make safe directory configurable

[waldner]

Currently, the code just sets /github/workspace (hardcoded) as "safe" git directory. I'm using the action in a non-github CI pipeline (forgejo) and I'm getting the error

fatal: detected dubious ownership in repository at '/workspace/waldner/cert-manager-webhook-he'
To add an exception for this directory, call:
	git config --global --add safe.directory /workspace/waldner/cert-manager-webhook-he

So I'm submitting this PR with a proposal to make that directory configurable using SAFE_DIRECTORY (the default, of course, still being /github/workspace).

[waldner]

Modified to use $GITHUB_WORKSPACE instead of introducing a new variable.