Assumed location of git repository should be configurable

[LebedevRI]

Currently, the action expects that actions/checkout@v2 was run with default with: path: '',
and if the repo was checked out into some directory, the codeql-action/init fails to find it:
https://github.com/LebedevRI/rawspeed/runs/1396938248?check_suite_focus=true#step:8:56

Run github/codeql-action/init@v1
/usr/bin/docker exec  78986cf722c833ce17429d007c42a3ccb6620eaba4a72ba2d33d11250d91b76b sh -c "cat /etc/*release | grep ^ID"
fatal: not a git repository (or any parent up to mount point /)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
Failed to call git to get current commit. Continuing with data from environment: Error: The process 'git' failed with exit code 128

[btashton]

This is making it more difficult for us to use this at https://github.com/apache/incubator-nuttx since our setup checks out three repos next to each-other and then issue the build. We can check these extra repos out inside of the primary one, but that causes other complications when we clean the build.