Makefile

The `test-with-reftable` CI job executes the "test-go" target, where the
only difference to our normal test target is that it also exports the
"GIT_DEFAULT_REF_FORMAT" environment variable. This environment variable
gets exported into all dependencies, which causes us to initialize repos
other than the test repositories created by our actual unit tests with
the "reftable" format. This includes for example the bundled Git repos.

This causes an issue with CI caches: it can happen that a job gets a CI
cache that uses the "files" format for the bundled Git repos, but the
default ref format is "reftable". This causes the following error when
reinitializing the directoy:

    fatal: could not open '/builds/gitlab-org/gitaly/_build/deps/git-v2.48/.git/refs/heads' for writing: Is a directory

The issue here is that git-init(1) tries to reinitialize the repository
with the wrong ref format. When doing so it wants to write "refs/heads"
as a file containing garbage to keep other Git clients which aren't yet
aware of the "reftable" format from reading the repository. But because
the repo uses the "files" format the path is a directory, and thus
writing the file fails.

The issue can be trivially reproduced with an up-to-date Git version:

    $ git init --ref-format=files repo
    Initialized empty Git repository in /tmp/repo/.git/
    $ GIT_DEFAULT_REF_FORMAT=reftable git init repo
    fatal: could not open '/tmp/repo/.git/refs/heads' for writing: Is a directory

Even though this is an upstream bug, it still doesn't make sense for us
to propagate the `GIT_DEFAULT_REF_FORMAT` environment variable into all
build prerequisites. Fix this by using a Gitaly-specific environment
variable instead and setting it up when configuring our tests.