Use recommended pattern for Gradle actions and side-by-side projects. (…

…#128)

Set up Gradle in one step and just run it as normal run steps. See
[docs](https://github.com/gradle/gradle-build-action?tab=readme-ov-file#use-the-action-to-setup-gradle).

Also check out `jspecify-reference-checker` and `jspecify` side-by-side
within the workspace (see
[docs](https://github.com/actions/checkout?tab=readme-ov-file#checkout-multiple-repos-side-by-side)),
instead of putting `jspecify` and the other siblings _next to_ the
workspace.

.github/workflows/build.yml

@@ -6,25 +6,29 @@ jobs:
   build-and-test:
     name: Build and Test
     runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: jspecify-reference-checker
     steps:
       - name: Check out the code
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
+        with:
+          path: jspecify-reference-checker
       - name: Set up Java
         uses: actions/setup-java@v3
         with:
           distribution: temurin
           java-version: 17
-      - name: Build and Test
+      - name: Set up Gradle
         uses: gradle/gradle-build-action@v2
-        with:
-          arguments: build conformanceTests demoTest
+      - name: Build and Test
+        run: ./gradlew build conformanceTests demoTest
         env:
           SHALLOW: 1
       - name: Check out jspecify/samples-google-prototype
         if: always()
-        run: git -C ../jspecify checkout samples-google-prototype
+        run: git checkout samples-google-prototype
+        working-directory: jspecify
       - name: Run Samples Tests
         if: always()
-        uses: gradle/gradle-build-action@v2
-        with:
-          arguments: jspecifySamplesTest
+        run: ./gradlew jspecifySamplesTest