CI: Checkout driver tag into the directory we build from

It was previously checking out the default branch into the $DRIVER
subdirectory, and the requested tag into the current subdirectory,
but then proceeding to build the $DRIVER subdirectory.

Fixes: 89b7f4501 ("CI: add a driver build stage to check for header breakage")
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/1607>
(cherry picked from commit 552250901f)

.gitlab-ci.yml

@@ -200,7 +200,7 @@ xf86-driver-build-test:
         - unset MESON_EXTRA_ARGS
         - DRIVER=$(basename $REPO)
         - git clone "$REPO" "$DRIVER"
-        - GIT_DIR="$DRIVER/.git" git checkout -f "$SHA"
+        - git -C "$DRIVER" checkout "$SHA"
         - |
           if [[ -e "$DRIVER/meson.build" ]]; then
             .gitlab-ci/meson-build.sh -C "$DRIVER" --skip-test