feat(eval): Mount depot_tools in sandbox

When sandboxing, automatically find and mount the `depot_tools`
directory. This ensures essential development tools are available.

If `depot_tools` cannot be located, the provider now returns a clear
error instead of proceeding with a misconfigured sandbox.

Bug: 448648646
Change-Id: If544709a3a458cc180240874a64e6ba617bd6102
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7013810
Auto-Submit: James Woo <jewoo@google.com>
Reviewed-by: Struan Shrimpton <sshrimp@google.com>
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1526378}