Startup script solution gets stuck in infinite loop

#### Overview of the Issue

If the Packer VM is:

- configured to use startup-script metadata to perform customization
- given a service account that does not have the power to modify its own metadata

then the `packer` process gets stuck in an [infinite loop](https://github.com/hashicorp/packer-plugin-googlecompute/blob/289eb5985bbfefc37b03743d6a6d0a8b557643c8/builder/googlecompute/step_wait_startup_script.go#L40-L48). The guidance to the user is not very informative. My thoughts:

1. modify `retry.Config` to put a limit on the number of Tries or StartTimeout
2. Improve the guidance to the user at "Error getting startup script status" to help them understand that the service account probably needs the permission to modify its own instance metadata
3. Whatever process attempts to update the instance metadata should probably have a retry mechanism

These could be done separately. 1 and 2 are probably obvious. The reasoning behind 3 may not be. If you create a service account on Google Cloud and assign it IAM roles, those roles are not immediately applied but have a [known propagation delay](https://cloud.google.com/iam/docs/access-change-propagation). Thus an automation pipeline might create the service account, assign it adequate permissions, and nevertheless Packer might fail.

Each timeout might reasonably be 10 minutes to account for worst case propagation delay.

#### Reproduction Steps

Begin by creating a service account without any IAM roles:

```shell
gcloud iam service-accounts create failure \
--description="SA" \
--display-name="failure"
```

Then supply that project_id and service account to the template below.

### Plugin and Packer version

- Packer v1.10.2
- Plugin latest as shown below

### Simplified Packer Buildfile

```hcl
source "googlecompute" "toolkit_image" {
project_id = var.project_id
communicator = "none"
image_name = "repro-fail"
machine_type = "n2-standard-8"
disk_size = 32
disk_type = "pd-balanced"
omit_external_ip = true
use_internal_ip = true
subnetwork = "default"
zone = "us-central1-c"
service_account_email = var.service_account_email
scopes = ["https://www.googleapis.com/auth/cloud-platform"]
source_image_family = "debian-12"
metadata = {
startup-script = <<-EOD
#!/bin/bash
/bin/true
EOD
}
}

build {
name = "test"
sources = ["sources.googlecompute.toolkit_image"]
}

variable "project_id" {
description = "Project in which to create VM and image"
type = string
}

variable "service_account_email" {
description = "Service account email address"
type = string
}

packer {
required_version = ">= 1.7.9, < 2.0.0"

# packer plugin 1.0.16 and above includes HPC VM Image
required_plugins {
googlecompute = {
version = "~> 1.1.0"
source = "github.com/hashicorp/googlecompute"
}
}
}
```

### Log Fragments and crash.log files

```
tpdownes@poreef ~/repro> packer build -var project_id=my-project -var service_account_email=failure@my-project.iam.gserviceaccount.com .
test.googlecompute.toolkit_image: output will be in this color.

==> test.googlecompute.toolkit_image: Checking image does not exist...
==> test.googlecompute.toolkit_image: Creating temporary RSA SSH key for instance...
==> test.googlecompute.toolkit_image: no persistent disk to create
==> test.googlecompute.toolkit_image: Using image: debian-12-bookworm-v20240312
==> test.googlecompute.toolkit_image: Creating instance...
test.googlecompute.toolkit_image: Loading zone: us-central1-c
test.googlecompute.toolkit_image: Loading machine type: n2-standard-8
test.googlecompute.toolkit_image: Requesting instance creation...
test.googlecompute.toolkit_image: Waiting for creation operation to complete...
test.googlecompute.toolkit_image: Instance has been created!
==> test.googlecompute.toolkit_image: Waiting for the instance to become running...
test.googlecompute.toolkit_image: IP: 10.128.0.10
==> test.googlecompute.toolkit_image: Waiting for any running startup script to finish...
test.googlecompute.toolkit_image: Metadata startup-script-status on instance packer-6615be2c-4509-e09b-a563-a2a3fcc15cf6 not available. Waiting...
test.googlecompute.toolkit_image: Metadata startup-script-status on instance packer-6615be2c-4509-e09b-a563-a2a3fcc15cf6 not available. Waiting...
test.googlecompute.toolkit_image: Metadata startup-script-status on instance packer-6615be2c-4509-e09b-a563-a2a3fcc15cf6 not available. Waiting...
test.googlecompute.toolkit_image: Metadata startup-script-status on instance packer-6615be2c-4509-e09b-a563-a2a3fcc15cf6 not available. Waiting...
test.googlecompute.toolkit_image: Metadata startup-script-status on instance packer-6615be2c-4509-e09b-a563-a2a3fcc15cf6 not available. Waiting...
test.googlecompute.toolkit_image: Metadata startup-script-status on instance packer-6615be2c-4509-e09b-a563-a2a3fcc15cf6 not available. Waiting...
test.googlecompute.toolkit_image: Metadata startup-script-status on instance packer-6615be2c-4509-e09b-a563-a2a3fcc15cf6 not available. Waiting...
test.googlecompute.toolkit_image: Metadata startup-script-status on instance packer-6615be2c-4509-e09b-a563-a2a3fcc15cf6 not available. Waiting...
test.googlecompute.toolkit_image: Metadata startup-script-status on instance packer-6615be2c-4509-e09b-a563-a2a3fcc15cf6 not available. Waiting...
test.googlecompute.toolkit_image: Metadata startup-script-status on instance packer-6615be2c-4509-e09b-a563-a2a3fcc15cf6 not available. Waiting...
test.googlecompute.toolkit_image: Metadata startup-script-status on instance packer-6615be2c-4509-e09b-a563-a2a3fcc15cf6 not available. Waiting...
test.googlecompute.toolkit_image: Metadata startup-script-status on instance packer-6615be2c-4509-e09b-a563-a2a3fcc15cf6 not available. Waiting...
test.googlecompute.toolkit_image: Metadata startup-script-status on instance packer-6615be2c-4509-e09b-a563-a2a3fcc15cf6 not available. Waiting...
test.googlecompute.toolkit_image: Metadata startup-script-status on instance packer-6615be2c-4509-e09b-a563-a2a3fcc15cf6 not available. Waiting...
test.googlecompute.toolkit_image: Metadata startup-script-status on instance packer-6615be2c-4509-e09b-a563-a2a3fcc15cf6 not available. Waiting...
test.googlecompute.toolkit_image: Metadata startup-script-status on instance packer-6615be2c-4509-e09b-a563-a2a3fcc15cf6 not available. Waiting...
Cancelling build after receiving interrupt
test.googlecompute.toolkit_image: Metadata startup-script-status on instance packer-6615be2c-4509-e09b-a563-a2a3fcc15cf6 not available. Waiting...
==> test.googlecompute.toolkit_image: Error waiting for startup script to finish: Error getting startup script status: Instance metadata key, startup-script-status, not found.
```