Add GHA cache (#1604)

Author: trask

.github/workflows/pr.yaml

@@ -30,20 +30,24 @@ jobs:
       fail-fast: false
     steps:
       - uses: actions/checkout@v2
-      - id: setup-test-java
-        name: Set up JDK 11 for running Gradle
+      - name: Set up Java 11
         uses: actions/setup-java@v1
         with:
           java-version: 11
-      - name: Build otel-fork
+      - name: Init and update otel submodule
         run: |
           git submodule init
           git submodule update
+      - name: Restore cache
+        uses: burrunan/gradle-cache-action@v1
+      - name: Build otel-fork
+        run: |
           cd otel
           # TODO why does this error only happen in GHA, without "-x signMavenPublication"?
           #   "Cannot perform signing task ':instrumentation-api:signMavenPublication' because it has no configured signatory"
           ./gradlew publishToMavenLocal -x signMavenPublication
       - name: Test
+        # TODO enable build cache, either --build-cache here, or org.gradle.caching=true in gradle.properties
         run: ./gradlew ${{ matrix.module }}:smokeTest --no-daemon
 
   accept-pr: