fix: cache Robolectric SDK artifacts in CI to prevent test hang

Robolectric downloads ~350MB of Android SDK binaries to ~/.m2/repository/org/robolectric
on first run. The CI cache only covered ~/.gradle, so cache misses caused a silent
multi-hour download mid-test-run that looked like a hang.

Author: turnipdabeets

.github/workflows/build.yml

@@ -44,6 +44,7 @@ jobs:
           path: |
             ~/.gradle/caches
             ~/.gradle/wrapper
+            ~/.m2/repository/org/robolectric
           key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle*', '**/gradle-wrapper.properties') }}
           restore-keys: |
             ${{ runner.os }}-gradle-