testcfg: Fix building Go binaries with Go 1.18

Our testcfg package needs to build some of our Go binaries ad-hoc to make them available to our testing infrastructure. With Go 1.18 this functionality is broken though: Go tries to automatically detect VCS information to embed it into the resulting binaries. But because we're intercepting all Git invocations that are resolved via the system's PATH variable in order to verify that we don't ever do that, Go will also use that intercepting script and consequentially get an error.

Fix this regression by setting up a Git command factory and tweaking the environment to make Go use its binaries.

Changelog: fixed

Fixes #4178 (closed)