ci: add git to the CI image

This allows the CDash submission to know what commit is being submitted
and show up in MR pipeline status links made by our workflow processes.

.gitlab/ci/docker/gcc_cxx_modules/install_deps.sh

@@ -3,5 +3,5 @@
 set -e
 
 dnf install -y --setopt=install_weak_deps=False \
-    gcc-c++ mpfr-devel libmpc-devel isl-devel flex bison file findutils diffutils
+    gcc-c++ mpfr-devel libmpc-devel isl-devel flex bison file findutils diffutils git-core
 dnf clean all