ci: Fix Git patches being applied on top of non-default versions

We have recently started to grow a set of Git patches on top of our own Git version. While the intention is that those patches are only applied for the default Git version of ours, where Gitaly must support Git with and without these patches, we have indeed been applying those patches to all our currently tested Git versions. In fact, it's by pure chance only that all patches apply to v2.31.1, v2.32.0 and v2.33.0.

We're about to add some patches which do not apply on the older Git versions and which thus break those builds. This commit thus fixes the CI job definitions for non-v2.33.0 version by explicitly overriding the GIT_PATCHES variable such that our Makefile won't set up the default set of patches anymore.