check: Ensure unpacked binaries are cleaned up

Addresses #6413

The gitaly check command instantiates a GitLabHookManager to verify that Gitaly is able to access Rails' /internal/check endpoint. The hook manager requires an ExecCommandFactory which requires Git binaries to be configured properly. Due to this, we must unpack the embedded Git binaries into cfg.RuntimeDir when this command is executed.

There were two problems with the implementation:

• We didn't clean up the runtime directory, so multiple invocations of the command would fail when it tries to unpack binaries into an existing location.
• We use cfg.RuntimeDir directly as configured in config.toml. We should instead be creating subdirectories like we do elsewhere.

Address these problems by unpacking binaries into a subdirectory of cfg.RuntimeDir, and ensure the subdirectory is cleaned up at the end of execution