Fix Windows storage issues and Linux depot_tools init

1. Windows: Remove explicit switch to `C:` drive in `build_windows.ps1`. The runner's `D:` drive has ~150GB free vs `C:`'s ~36GB, so building on `D:` (default) resolves "No space left on device" errors.
2. Windows: Add disk cleanup step in workflow to free `C:` space (Android, SQL, etc.).
3. Linux: Set `USER=builder` and prepend `depot_tools` to `PATH` in `build_linux.sh` to fix initialization failures.

Author: google-labs-jules[bot]

windows/build_windows.ps1

@@ -4,8 +4,7 @@ Write-Output "::group::Init"
 
 $WorkflowStartDir = (Get-Location).Path
 
-# The C:\ drive has about 80 GB, the D: drive (default) only 12. We need about 25 GB, so we switch over
-Set-Location C:\
+# The D: drive (default) has more space (~150GB) than C: (~113GB used, 36GB free), so we stay on D:.
 function New-TemporaryDirectory {
     $parent = [System.IO.Path]::GetTempPath()
     [string] $name = [System.Guid]::NewGuid()