[main] Fix CG pipeline permission error: use writable install path

[Copilot]

The dotnet-docker-cg-code pipeline fails with permission denied when creating /usr/share/.dotnet on the Linux pool image.

Changes

• Update eng/docker-tools/templates/jobs/cg-build-projects.yml to use $(Build.SourcesDirectory)/.dotnet instead of /usr/share/.dotnet

Note

This file is in eng/docker-tools/ — changes should be backported to dotnet/docker-tools.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Pipeline Failure: dotnet-docker-cg-code - Permission denied when creating .NET install directory</issue_title>
<issue_description>### Description

The dotnet-docker-cg-code pipeline is failing during the "Run Dotnet Install Script" step with a permission denied error when attempting to create the .dotnet directory.

Error

/bin/mkdir: cannot create directory '/usr/share/.dotnet': Permission denied
Executing 'Invoke-WebRequest 'https://builds.dotnet.microsoft.com/dotnet/scripts/v1/dotnet-install.sh' -OutFile /usr/share/.dotnet/dotnet-install.sh'
Retry 1/2, retrying in 1 seconds...
Retry 2/2, no more retries left.
Exception: Failed to execute 'Invoke-WebRequest 'https://builds.dotnet.microsoft.com/dotnet/scripts/v1/dotnet-install.sh' -OutFile /usr/share/.dotnet/dotnet-install.sh'

Root Cause

The Install-DotNetSdk.ps1 script attempts to create the install directory at /usr/share/.dotnet, but the pipeline agent doesn't have write permissions to /usr/share/.

The failing code is at line 29:

if (!(Test-Path "$InstallPath")) {
    mkdir "$InstallPath" | Out-Null
}

Suggested Fix

Change the install path to a directory the agent has permission to write to (e.g., $HOME/.dotnet or a path under the agent's working directory).

Pipeline Information

• Pipeline: dotnet-docker-cg-code
• Failing Step: 🟪 Run Dotnet Install Script
• Build: https://dev.azure.com/dnceng/internal/_build/results?buildId=2873939&view=logs&j=3d756d90-2ba6-5bcd-7b25-4a1d533a722f</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Pipeline Failure: dotnet-docker-cg-code - Permission denied when creating .NET install directory #6907

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[lbussell]

I verified this works in build#2873967

[lbussell]

/backport to nightly

[github-actions]

Started backporting to nightly (link to workflow run)

[lbussell]

This belongs in the docker-tools repo instead.