GHA: drop actionlint, abandoned upstream

No release in 6 months, no commit in 5 months. Lots of debt and a
matching number of local workarounds.

Ref: https://github.com/rhysd/actionlint
Follow-up to f61703cc98 #1740

Closes #2636

.github/workflows/ci.yml

@@ -48,7 +48,7 @@ jobs:
 
       - name: 'install prereqs'
         run: |
-          /home/linuxbrew/.linuxbrew/bin/brew install actionlint shellcheck zizmor
+          /home/linuxbrew/.linuxbrew/bin/brew install shellcheck zizmor
           python3 -m venv ~/venv
           ~/venv/bin/pip --disable-pip-version-check --no-input --no-cache-dir install --progress-bar off --prefer-binary -r .github/workflows/requirements.txt
 
@@ -64,16 +64,6 @@ jobs:
           eval "$(/home/linuxbrew/.linuxbrew/bin/brew shellenv)"
           zizmor --pedantic .github/workflows/*.yml .github/dependabot.yml
 
-      - name: 'actionlint'
-        run: |
-          eval "$(/home/linuxbrew/.linuxbrew/bin/brew shellenv)"
-          actionlint --version
-          export SHELLCHECK_OPTS='--exclude=1090,1091,2086,2153 --enable=avoid-nullary-conditions,deprecate-which'
-          actionlint \
-            --ignore ubuntu-26.04 \
-            --ignore 'unexpected key "cache-mode"' \
-            .github/workflows/*.yml
-
       - name: 'shellcheck GHA'
         run: |
           eval "$(/home/linuxbrew/.linuxbrew/bin/brew shellenv)"