ci: various fixes

- install attr only when tesitng
- fix s390x workflow by checking out the merge PR instead of master

(cherry picked from commit 9ad239690f)

.github/workflows/optional.yml

@@ -7,9 +7,14 @@ concurrency:
   group: ${{ github.workflow }}-${{ github.event_name == 'pull_request' && github.head_ref || github.sha }}
   cancel-in-progress: true
 
+env:
+  INSTALL_PREFIX: ${{ github.workspace }}/nvim-install
+  # TEST_FILE: test/functional/shada
+  # TEST_FILTER: foo
+
 jobs:
   s390x:
-    if: ${{ github.event.label.name == 'ci-s390x' }}
+    if: contains(github.event.pull_request.labels.*.name, 'ci-s390x')
     strategy:
       fail-fast: false
       matrix:
@@ -24,9 +29,11 @@ jobs:
             bash -c
             "
             apt-get -y update &&
-            DEBIAN_FRONTEND=noninteractive apt-get -y install attr build-essential cmake curl gettext libattr1-dev ninja-build unzip locales-all cpanminus git &&
+            DEBIAN_FRONTEND=noninteractive apt-get -y install build-essential cmake curl gettext ninja-build unzip locales-all cpanminus git attr libattr1-dev  &&
             git clone --depth=1 https://github.com/neovim/neovim.git &&
             cd neovim &&
+            git fetch origin ${{ github.ref }}:pr &&
+            git switch pr &&
             cmake -S cmake.deps -B .deps -G Ninja -D USE_BUNDLED_LUAJIT=OFF -D USE_BUNDLED_LUA=ON &&
             cmake --build .deps &&
             cmake -B build -G Ninja -D PREFER_LUA=ON &&