fmf: Insist on mkosi sha being available

Let's assume the workflow file containing the mkosi always being
available.

Also fix the detection while we're at it by not leaving the systemd
directory.

test/fmf/integration-tests/test.sh

@@ -17,20 +17,14 @@ if [[ -n "${PACKIT_SOURCE_URL:-}" ]]; then
     git merge "pr/$PACKIT_SOURCE_BRANCH"
 fi
 git log --oneline -5
-popd
 
 # Now prepare mkosi, possibly at the same version required by the systemd repo
 git clone https://github.com/systemd/mkosi
-# If we have it, pin the mkosi version to the same one used by Github Actions, to ensure consistency
-if [ -f .github/workflows/mkosi.yml ]; then
-    mkosi_hash="$(grep systemd/mkosi@ .github/workflows/mkosi.yml | sed "s|.*systemd/mkosi@||g")"
-    git -C mkosi checkout "$mkosi_hash"
-fi
+mkosi_hash="$(grep systemd/mkosi@ .github/workflows/mkosi.yml | sed "s|.*systemd/mkosi@||g")"
+git -C mkosi checkout "$mkosi_hash"
 
 export PATH="$PWD/mkosi/bin:$PATH"
 
-pushd systemd
-
 # shellcheck source=/dev/null
 . /etc/os-release || . /usr/lib/os-release