[Q] would it be bad to make /etc/gitconfig runtime configurable?

The subject says it all. Would it be bad if we introduce an environment variable, GIT_SYSTEM_CONFIG=/etc/gitconfig, that names an alternative location of the system-wide configuration file?

That would supersede/deprecate GIT_CONFIG_NOSYSTEM that we introduced primarily so that we can run our tests without getting affected by the configuration that happens to be effective on the host that the test is being run.