ci: Split up test and build steps

.github/composite/gradle/action.yml

@@ -0,0 +1,27 @@
+# SPDX-FileCopyrightText: 2024 Linnea Gräf <nea@nea.moe>
+#
+# SPDX-License-Identifier: CC0-1.0
+
+name: "Run Gradle on a filter=tree:0 checkout"
+runs:
+    using: composite
+    steps:
+        -   uses: actions/checkout@v4
+            name: Checkout repository
+            with:
+                fetch-tags: true
+                fetch-depth: 0
+                filter: 'tree:0'
+        -   name: Set up JDK 21
+            uses: actions/setup-java@v4
+            with:
+                distribution: temurin
+                java-version: 21
+        -   name: Set up gradle cache
+            uses: gradle/actions/setup-gradle@v3
+            with:
+                add-job-summary-as-pr-comment: 'on-failure'
+        -   name: Prepare unpacked Jars
+            run: |
+                ./gradlew unpackAllJars
+            shell: sh