feat: shallow clone action repositories (#1053)

## Summary

When a workflow references a remote action (e.g. `uses: actions/checkout@v4`) the runner clones that repository during job setup.
Previously this was always a full clone(every branch and the complete history) even though only a single ref is needed.

This PR makes the runner shallow-clone the requested ref by default (`--depth=1`, single branch), falling back to a full clone when a shallow clone fails.

Notes:
- Existing on-disk caches are reused as-is; there is no forced re-clone on upgrade.

## Changes

- A new `runner.action_shallow_clone` option (default `true`) lets operators opt back into full clones.
- `cloneAtDepth`: attempt a shallow clone; fall back to a full clone when shallow clone fails.
- Keep a shallow cache cheap on update: fetch the single requested ref at depth 1 and skip `pull`.

---------

Co-authored-by: bircni <bircni@icloud.com>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1053
Reviewed-by: Nicolas <bircni@icloud.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-committed-by: Zettat123 <zettat123@gmail.com>

internal/pkg/config/config.example.yaml

@@ -69,6 +69,9 @@ runner:
   # and github_mirror is not empty. In this case,
   # it replaces https://github.com with the value here, which is useful for some special network environments.
   github_mirror: ''
+  # When true (the default), fetch only the requested ref of an action repository (e.g. actions/checkout@v4) at depth 1 instead of cloning every branch's full history. 
+  # Set to false to clone the full history.
+  action_shallow_clone: true
   # The labels of a runner are used to determine which jobs the runner can run, and how to run them.
   # Like: "macos-arm64:host" or "ubuntu-latest:docker://docker.gitea.com/runner-images:ubuntu-latest"
   # Find more images provided by Gitea at https://gitea.com/gitea/runner-images .