IAP Cloud Build: SSH Permission Denied

#### Overview of the Issue

I am trying to use Google Cloud Build to create Packer images, but with the machines being configured to have no external IP. The IAP options seem perfect for this, but I have yet to get a standard build to finish with those options being set. It always fails when trying to establish the SSH connection.

I have verified that it is not a permissions issue with the firewall rules or the service account IAM permissions as I can impersonate it on the CLI and there is no problem connecting through SSH to the private machines.

#### Reproduction Steps

1. Prepare a Google Cloud Build run, with the relevant steps to execute Packer with this plugin (example below).
2. Supply Packer with the config file described below (I do it through a cloud source repo).
3. Assign the proper permissions to the cloud build service account (I use a custom one). Most notable is `IAP-secured Tunnel User` but others will probably be required.
4. Configure your VPC firewall rules to allow the IAP CIDR (35.235.240.0/20) into all the instances in the VPC.
5. Trigger the build step and see it hang and timeout on this message: "Waiting for SSH to become available...".

### Plugin and Packer version

Plugin: 1.0.13
Packer: Docker image, light-1.8.2

### Simplified Packer Buildfile
Note: I have tried with a dynamic and static IAP localhost port, with no setting making a difference.
```hcl
source "googlecompute" "base-deb11" {
source_image = "a custom Debian 11 image"
enable_secure_boot = true
enable_vtpm = true
enable_integrity_monitoring = true

network = "network"
subnetwork = "subnet"
omit_external_ip = true
use_internal_ip = true
use_iap = true
# iap_localhost_port = 22

service_account_email = "cloud-build@${var.project_id}.iam.gserviceaccount.com"
impersonate_service_account = "cloud-build@${var.project_id}.iam.gserviceaccount.com"

temporary_key_pair_type = "ed25519"
ssh_username = "auto-replaced"
use_os_login = false
}
```

### Operating system and Environment details
- Running in Google Cloud Build. I have tried both the default worker space and private worker pools.
- VM image is a lightly modified version of Debian 11.3. The firewall allows SSH (port 22) and works fine when an external IP is attached, so this is also not the issue.

```yml
source:
repoSource:
projectId: projects/redacted
repoName: redacted
branchName: base-deb11
dir: base-deb11
steps:
- name: 'hashicorp/packer:light'
args:
- '-c'
- packer init base-deb11.gcloud.pkr.hcl
dir: /workspace/base-deb11
entrypoint: bash
- name: 'hashicorp/packer:light'
env:
- PACKER_LOG=1
- PACKER_LOG_PATH=/workspace/packer.log
args:
- '-c'
- packer build base-deb11.gcloud.pkr.hcl
dir: /workspace/base-deb11
entrypoint: bash
```

### Log Fragments and crash.log files
These are the relevant lines from packer.log, with the entire gist still being [linked](https://gist.github.com/oinkbark/9bc2c7a13fb50fe9965c73c6d01a16cd):

```
2022/06/30 00:04:08 packer-plugin-googlecompute_v1.0.13_x5.0_linux_amd64 plugin: 2022/06/30 00:04:08 Finding an available TCP port for IAP proxy
2022/06/30 00:04:08 packer-plugin-googlecompute_v1.0.13_x5.0_linux_amd64 plugin: 2022/06/30 00:04:08 Found available port: 8290 on IP: 0.0.0.0
2022/06/30 00:04:08 packer-plugin-googlecompute_v1.0.13_x5.0_linux_amd64 plugin: 2022/06/30 00:04:08 Setting up proxy to listen on localhost at 8290
2022/06/30 00:04:08 packer-plugin-googlecompute_v1.0.13_x5.0_linux_amd64 plugin: 2022/06/30 00:04:08 Creating tunnel launch script with args []string{"compute", "start-iap-tunnel", "base-deb11", "22", "--local-host-port=localhost:8290", "--zone", "redacted", "--project", "redacted", "--impersonate-service-account='cloud-build@redacted.iam.gserviceaccount.com'"}
2022/06/30 00:04:08 packer-plugin-googlecompute_v1.0.13_x5.0_linux_amd64 plugin: 2022/06/30 00:04:08 [INFO] (google): Prepending inline gcloud setup script with #!/bin/sh
2022/06/30 00:04:09 packer-plugin-googlecompute_v1.0.13_x5.0_linux_amd64 plugin: 2022/06/30 00:04:09 stderr: /tmp/gcloud-setup3357350656: line 2: gcloud: not found
2022/06/30 00:04:38 packer-plugin-googlecompute_v1.0.13_x5.0_linux_amd64 plugin: 2022/06/30 00:04:38 No error detected after tunnel launch; continuing...
2022/06/30 00:04:38 packer-plugin-googlecompute_v1.0.13_x5.0_linux_amd64 plugin: 2022/06/30 00:04:38 Using host value: localhost
2022/06/30 00:04:38 ui: ==> googlecompute.base-deb11: Using SSH communicator to connect: localhost
2022/06/30 00:04:38 packer-plugin-googlecompute_v1.0.13_x5.0_linux_amd64 plugin: 2022/06/30 00:04:38 [INFO] Waiting for SSH, up to timeout: 5m0s
2022/06/30 00:04:38 ui: ==> googlecompute.base-deb11: Waiting for SSH to become available...
2022/06/30 00:04:38 packer-plugin-googlecompute_v1.0.13_x5.0_linux_amd64 plugin: 2022/06/30 00:04:38 Using host value: localhost
2022/06/30 00:04:38 packer-plugin-googlecompute_v1.0.13_x5.0_linux_amd64 plugin: 2022/06/30 00:04:38 [DEBUG] TCP connection to SSH ip/port failed: dial tcp 127.0.0.1:8290: connect: connection refused
```

### Potential Workarounds
Internally, it appears that this plugin is using this command, which is causing the issue:
```
gcloud compute start-iap-tunnel 22 --local-host-port=localhost: --zone --project --impersonate-service-account=''
```

I have been able to get the build to work using these two alternatives:
1. Using tunnel-though
```
gcloud compute ssh --tunnel-through-iap --project --zone
```
2. Using private worker pools. This replaces IAP (ie you disable it) and makes the cloud build node apart of a private worker pool that directly has access to the local IPs of the machines that it creates. This requires some extra firewall rules, IAM permissions, peering, and private service connections.