mach: use uv run --frozen (#42169)

This will avoid updating the lockfile during regular operations. Also
ignore the `servo.egg-info` directory, which
the new python requirement installation mechanism via setuptools
creates.
To update the lockfile one can run `uv lock`. This is also done
automatically, when syncing requirements from WPT.

This PR also updates the uv version in CI to a newer one. If you
encounter any issues with the lockfile after this PR, it might be
necessary to update your local instance of uv.

Testing: Build still works. [mach try
full](https://github.com/servo/servo/actions/runs/21624364040)

---------

Signed-off-by: Jonathan Schwender <schwenderjonathan@gmail.com>

mach.ps1

@@ -23,4 +23,4 @@ if ($arguments.Count -gt 0) {
     }
 }
 
-uv run python (Join-Path $workdir "mach") @arguments
+uv run --frozen python (Join-Path $workdir "mach") @arguments