CI GitHub Actions: Cache JARs for Shadow CLJS (#16965)

Some front-end tasks require Shadow CLJS to compile files into
JavaScript, therefore let's cache the JARs which it needs.

.github/workflows/frontend.yml

@@ -49,6 +49,11 @@ jobs:
       uses: actions/setup-node@v1
       with:
         node-version: 14.x
+    - name: Get M2 cache
+      uses: actions/cache@v2
+      with:
+        path: ~/.m2
+        key: ${{ runner.os }}-cljs-${{ hashFiles('**/shadow-cljs.edn') }}
     - name: Get yarn cache
       uses: actions/cache@v2
       with:
@@ -85,6 +90,11 @@ jobs:
       uses: actions/setup-node@v1
       with:
         node-version: 14.x
+    - name: Get M2 cache
+      uses: actions/cache@v2
+      with:
+        path: ~/.m2
+        key: ${{ runner.os }}-cljs-${{ hashFiles('**/shadow-cljs.edn') }}
     - name: Get yarn cache
       uses: actions/cache@v2
       with:
@@ -103,6 +113,11 @@ jobs:
       uses: actions/setup-node@v1
       with:
         node-version: 14.x
+    - name: Get M2 cache
+      uses: actions/cache@v2
+      with:
+        path: ~/.m2
+        key: ${{ runner.os }}-cljs-${{ hashFiles('**/shadow-cljs.edn') }}
     - name: Get yarn cache
       uses: actions/cache@v2
       with: