mesa-snapshot: actually checkout selected commit id

The code in the mesa-snapshot.sh was fetching the specified commit, but
wasn't switching to it. It wasn't that important previously, as
packages were simply using newer commit, but it's important now as we
are using 25.2 branch instead of main.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>

Author: lumag

overlay-debs/mesa-snapshot/mesa-snapshot.sh

@@ -17,6 +17,8 @@ git clone --depth 1 https://gitlab.freedesktop.org/mesa/mesa
 cd mesa
 git fetch --depth 1 origin ${COMMIT##origin/}
 
+git checkout -f ${COMMIT}
+
 date=$(git log -1 --format=%cd --date=format:%Y%m%d ${COMMIT})
 subject=$(git log -1 --format="%h (\"%s\")" ${COMMIT})
 version=25.2.0~git${date}