feat: initial implementation

[cgrindel]

• The initial implementation for the action.
  • /tools/run_nix_shell.sh: The core logic for the action.
  • /action.yaml: The GitHub action definition.
• Nix configuration files to support the testing of the action (/flake.nix, /flake.lock, /nixpkgs.nix).
• Unit tests (/tests/tools_tests) that can be exercised using Bazel.
• Files used in the integration tests (/tests/integration_tests). The actual integration tests are defined in the CI workflow file.
• A CI workflow to test PRs and run a daily build.
• README.md with usage information.

Related to https://github.com/tweag/scalable-builds-group/issues/97. Related to https://github.com/tweag/scalable-builds-group/issues/138.

[cgrindel]

@avdv This ended up being bigger than I originally thought it would be. I am willing to break it up into smaller PRs if you prefer. Just let me know.