fstests.yml: actually use the short ref

In the previous commit we defined a short ref but didn't use it, use
the short character version of it.

Signed-off-by: Luis Chamberlain <[email protected]>

Author: mcgrof

.github/workflows/fstests.yml

@@ -28,7 +28,7 @@ jobs:
         run: |
           KDEVOPS_TREE_REF="${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}"
           SHORT_PREFIX="$(echo ${KDEVOPS_TREE_REF:0:4})"
-          make KDEVOPS_HOSTS_PREFIX="$KDEVOPS_TREE_REF" defconfig-xfs_reflink_4k
+          make KDEVOPS_HOSTS_PREFIX="$SHORT_PREFIX" defconfig-xfs_reflink_4k
 
       - name: Run kdevops make
         run: |