Build latest Caffeine on CI (#1111)

Fixes #1110 

We can see how this goes for some time before making it a required job.

.github/workflows/caffeine-use-snapshot.gradle.kts

@@ -0,0 +1,23 @@
+// An init script to override Caffeine's build configuration to use a snapshot version of NullAway
+allprojects {
+  repositories {
+    mavenCentral()
+    mavenLocal()
+    gradlePluginPortal()
+  }
+}
+
+gradle.projectsLoaded {
+  rootProject.allprojects {
+    configurations.all {
+      resolutionStrategy {
+        eachDependency {
+          if (requested.group == "com.uber.nullaway") {
+            useVersion("+")
+          }
+        }
+        cacheChangingModulesFor(0, "seconds")
+      }
+    }
+  }
+}

.github/workflows/continuous-integration.yml

@@ -76,3 +76,30 @@ jobs:
         if: matrix.java == '17'
       - name: Check that Git tree is clean after build and test
         run: ./.buildscript/check_git_clean.sh
+  caffeine-gradle-task:
+    name: "Build Caffeine with snapshot"
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout this repository
+        uses: actions/checkout@v4
+      - name: 'Set up JDKs'
+        uses: actions/setup-java@v4
+        with:
+          java-version: 21
+          distribution: 'temurin'
+      - name: Setup Gradle
+        uses: gradle/actions/setup-gradle@v4
+      - name: Publish to Maven Local
+        env:
+          ORG_GRADLE_PROJECT_RELEASE_SIGNING_ENABLED: 'false'
+        run: ./gradlew publishToMavenLocal
+      - name: Create a temporary directory for Caffeine
+        run: mkdir -p /tmp/caffeine
+      - name: Clone Caffeine repository
+        run: git clone --depth 1 https://github.com/ben-manes/caffeine.git /tmp/caffeine
+      - name: Run caffeine build
+        run: |
+          cp .github/workflows/caffeine-use-snapshot.gradle.kts /tmp/caffeine
+          cd /tmp/caffeine
+          ./gradlew --init-script caffeine-use-snapshot.gradle.kts build -x test -x javadoc