CI: Cache dependencies

This will speed up our builds a bit and prevent unnecessary network traffic.

Setup according to the documentation for the `actions/cache@v2` action:
https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows

Currently caching the `${{github.workspace}}/_deps` directory, as used by CMake.

Cache keys look like this: `ubuntu-20.04-clang11` (`${{matrix.os}-${{matrix.compiler}}${matrix.compiler-version}}`)

PiperOrigin-RevId: 431895214
Change-Id: I4ecac7c00eec8516f85f45aa2220303b811b2389

.github/workflows/fedora-cmake.yml

@@ -12,7 +12,8 @@ jobs:
       matrix:
         include:
           - container: fedora:35
-            compiler: gcc  # GCC 11
+            compiler: gcc
+            compiler-version: 11  # Only used in cache action so far
             ignore-errors: true  # Stack trace test fails on Fedora (issue #118)
     runs-on: ubuntu-latest
     continue-on-error: ${{ matrix.ignore-errors }}
@@ -23,6 +24,13 @@ jobs:
     steps:
     - uses: actions/checkout@v2
 
+    - name: Cache dependencies
+      uses: actions/cache@v2
+      with:
+        key: ${{matrix.container}}-${{matrix.compiler}}${{matrix.compiler-version}}
+        path: |
+          ${{github.workspace}}/_deps
+
     - name: Prepare container
       # Note: For the sandbox tests to work, we need a privileged, unconfined
       #       container that retains its capabilities.

.github/workflows/ubuntu-cmake.yml

@@ -42,6 +42,13 @@ jobs:
     steps:
     - uses: actions/checkout@v2
 
+    - name: Cache dependencies
+      uses: actions/cache@v2
+      with:
+        key: ${{matrix.os}}-${{matrix.compiler}}${{matrix.compiler-version}}
+        path: |
+          ${{github.workspace}}/_deps
+
     - name: Install ninja-build tool
       uses: seanmiddleditch/gha-setup-ninja@v3