Can't get the config argument --builds-dir working.

I’m trying to understand how the `builds-dir` works with the Tart runner, and how to share a checked-out repository from the macOS host into a vanilla Tart VM.

**My goal is to:**

1. Job 1 (shell executor on host) - Clone the repository on the macOS host.
2. Job 2 (Tart executor) - Start a **vanilla macOS** VM and mount the previously cloned repository from Job 1 into the VM.

However, I’m running into issues with how the Tart executor mounts the `builds-dir`.

**The Problem**

Even though I configure a custom `--builds-dir` on the host `/Users/sysadmin/builds` the Tart executor always mounts it inside the VM as: `/var/tmp/builds`.

**Runner Configuration**

```
concurrent = 2
check_interval = 0
connection_max_age = "15m0s"
shutdown_timeout = 0

[session_server]
session_timeout = 1800

[[runners]]
name = "mac-mini-01-tart"
url = "https://foo.bar"
id = 123
token = "token-xyz"
executor = "custom"
[runners.feature_flags]
FF_RESOLVE_FULL_TLS_CHAIN = false
[runners.custom]
config_exec = "gitlab-tart-executor"
config_args = ["config", "--builds-dir", "/Users/sysadmin/builds"]
prepare_exec = "gitlab-tart-executor"
prepare_args = ["prepare"]
run_exec = "gitlab-tart-executor"
run_args = ["run"]
cleanup_exec = "gitlab-tart-executor"
cleanup_args = ["cleanup"]

[[runners]]
name = "mac-mini-01-shell"
url = "https://foo.bar"
id = 123
token = "token-xyz"
executor = "shell"
```

**.gitlab-ci.yml**
```
stages:
- checkout
- build

host-checkout:
stage: checkout
tags:
- macos-shell
script:
- echo "Repository checked out on host at $CI_PROJECT_DIR"
- ls -la

dotnet-dev:
stage: build
needs: [host-checkout]
variables:
GIT_STRATEGY: none
tags:
- tart-mac
image: ghcr.io/cirruslabs/macos-tahoe-vanilla:latest
script:
- echo "CI_PROJECT_DIR=$CI_PROJECT_DIR"
- ls -la "$CI_PROJECT_DIR"
```

**Job 1 Output (Shell Runner – Host)**
```
Running with gitlab-runner 18.7.1 (cc7f9277)
on mac-mini-01-shell UJ4PyLUA_, system ID: s_7d041a37a510
Resolving secrets
Preparing the "shell" executor
Using Shell (bash) executor...
Preparing environment
Running on GT-XMPJYXMXYG...
Getting source from Git repository
Gitaly correlation ID: 01KEAZK0EAX3YDRWJ5CW16VRN4
Fetching changes with git depth set to 50...
Reinitialized existing Git repository in /Users/sysadmin/builds/UJ4PyLUA_/1/dev/install/.git/
Checking out 137b3b7f as detached HEAD (ref is refs/merge-requests/158/head)...
Skipping Git submodules setup
Executing "step_script" stage of the job script
$ echo "Repository checked out on host at $CI_PROJECT_DIR"
Repository checked out on host at /Users/sysadmin/builds/UJ4PyLUA_/1/dev/install
$ ls -la
total 112
drwxr-xr-x@ 19 sysadmin staff 608 Jan 6 20:01 .
drwxr-xr-x@ 4 sysadmin staff 128 Jan 6 19:58 ..
-rwxr-xr-x@ 1 sysadmin staff 8881 Jan 6 19:58 bootstrap.sh
Cleaning up project directory and file based variables
Job succeeded
```

Job 2 Output (Tart Runner – VM)
```
Running with gitlab-runner 18.7.1 (cc7f9277)
on mac-mini-01-tart bcpX4Y6CR, system ID: s_7d041a37a510
Resolving secrets
Preparing the "custom" executor
Using Custom executor with driver tart 1.27.0-d5c86bf...
Pulling the latest version of ghcr.io/cirruslabs/macos-tahoe-vanilla:latest...
Cloning and configuring a new VM...
Waiting for the VM to boot and be SSH-able...
Was able to SSH!
Mounting buildsdir on /var/tmp/builds...
VM is ready.
Preparing environment
Running on Manageds-Virtual-Machine.local...
Getting source from Git repository
Skipping Git repository setup
Skipping Git checkout
Skipping Git submodules setup
Executing "step_script" stage of the job script
WARNING: Starting with version 17.0 the 'build_script' stage will be replaced with 'step_script': https://gitlab.com/groups/gitlab-org/-/epics/6112
$ echo "CI_PROJECT_DIR=$CI_PROJECT_DIR"
CI_PROJECT_DIR=/var/tmp/builds/dev/install
$ ls -la "$CI_PROJECT_DIR"
total 2
drwxr-xr-x 2 admin staff 64 Jan 7 00:45 .
drwxr-xr-x 4 admin staff 128 Jan 7 00:45 ..
```

**Environment**

Host macOS
```
ProductName: macOS
ProductVersion: 26.2
```

Installed tools:
```
gitlab-runner 18.7.1
gitlab-tart-executor 1.27.0
```

Any clarification or examples would be greatly appreciated. Thanks!

Start at the custom executor's config, prepare, run, and cleanup entry points named in the runner configuration, and trace how config_args handles --builds-dir before the VM reports its mount path. Compare the host checkout path with CI_PROJECT_DIR inside the VM; done means the intended shared repository behavior is implemented or the limitation is documented with a working configuration example.

Written by the indexing model from the issue text.