Bug 40064: Fix usage of exec without git_url/hg_url

Closes #40064 (closed).

This can be tested with tor-browser-build!845, which is using exec in a project without git/hg repo:

./rbm/rbm showconf --target alpha --target browser-all --target torbrowser release var/reproducible_build_infos

requested review from @richard

assigned to @boklm

approved this merge request

merged