Cache ctr_firm_builder.

.github/workflows/c-cpp.yml

@@ -19,17 +19,27 @@ jobs:
       with:
         fetch-depth: 1
         submodules: recursive
-    - name: Install dependencies
+
+    - name: Get ctr_firm_builder commit
+      run: echo "FIRM_BUILDER_COMMIT=$(git ls-remote https://github.com/derrekr/ctr_firm_builder.git HEAD | cut -f1)" >> $GITHUB_ENV
+
+    - name: Cache ctr_firm_builder
+      id: cache-firm-builder
+      uses: actions/cache@v3
+      with:
+        path: ctr_firm_builder
+        key: ${{ runner.os }}-${{ env.FIRM_BUILDER_COMMIT }}
+
+    - name: Install ctr_firm_builder dependencies
+      if: steps.cache-firm-builder.outputs.cache-hit != 'true'
       run: |
         apt-get update
         apt-get -y install gcc
 
     - name: Build ctr_firm_builder
-      uses: actions/checkout@v3
-      with:
-        repository: derrekr/ctr_firm_builder
-        path: ctr_firm_builder
+      if: steps.cache-firm-builder.outputs.cache-hit != 'true'
       run: |
+        git clone --depth 1 --recurse-submodules https://github.com/derrekr/ctr_firm_builder.git
         cd ctr_firm_builder
         make