Tests: build script doesn't set GIT_COMMIT... env variables when building from a downloaded source archive

[tranzystorekk]

I wanted to package sheldon for Void Linux, but the tests::version test fails to compile because the package is built from a downloaded .tar.gz source archive and so the build script is not run in a git repository and cannot set the GIT_COMMIT_SHORT_HASH and GIT_COMMIT_DATE variables.

[tranzystorekk]

Probably these two variables would have to be read in this test with option_env! instead, to provide the default "unknown" value when missing