ci: use local action to make make cache keys consistent (#20538)

The advantages of using an action is that it reduces boilerplate and
repetition from the main ci.yml workflow.

.github/actions/cache/action.yml

@@ -0,0 +1,22 @@
+name: 'Cache'
+description: "This action caches neovim dependencies"
+runs:
+  using: "composite"
+  steps:
+    - run: echo "CACHE_KEY=${{ github.job }}-${{ github.base_ref }}" >> $GITHUB_ENV
+      shell: bash
+
+    - if: ${{ matrix }}
+      run: echo "CACHE_KEY=$CACHE_KEY-${{ join(matrix.*, '-') }}" >> $GITHUB_ENV
+      shell: bash
+
+    # Avoid using '**/CMakeLists.txt' (or any pattern starting with '**/') even
+    # if it makes the expression below simpler. hashFiles() has a timer that
+    # will fail the job if it times out, which can happen if there are too many
+    # files to search through.
+    - uses: actions/cache@v3
+      with:
+        path: ${{ env.CACHE_NVIM_DEPS_DIR }}
+        key: ${{ env.CACHE_KEY }}-${{ hashFiles('cmake**', 'ci/**',
+          '.github/workflows/ci.yml', 'CMakeLists.txt',
+          'runtime/CMakeLists.txt', 'src/nvim/**/CMakeLists.txt') }}