In 08af0f9f (Speed up gitaly setup in tests, 2021-07-20) the setup of
Gitaly was sped up in test by copying it over from GDK whenever
possible. This drastically reduced the setup time, but it complicates
installations of Ruby gems in Gitaly.

This change rolls back that copy-from-GDK installation, but instead
incrementally installs Gitaly. So instead of removing tmp/tests/gitaly
completely and setting up from scratch, we now fetch the revision
specified in GITALY_SERVER_VERSION and trust `make` inside the Gitaly
directory to The Right Thing(tm).

Combined with the introduction of gitlab-development-kit@d2229da0
(Makefile: Stop nuking gitlab/tmp/tests/gitaly, 2021-12-06) this should
avoid Gitaly in test ever again would be built from scratch.

In case this would not work in some situations, we look at
FORCE_GITALY_INSTALL and if the environment variable is set
tmp/tests/gitaly is deleted before installing Gitaly again.