fix(ci): free disk space before building debug androidTest APKs

The scheduled 'Run balances tests' workflow has been failing for weeks
with 'No space left on device' — assembleDebug + assembleDebugAndroidTest
for every module exceeds the ~14 GB free on GitHub-hosted runners.

Add an opt-in free-disk-space input to the reusable build workflow that
removes preinstalled toolchains we never use (dotnet, ghc, CodeQL, swift,
boost, etc., ~30+ GB), and enable it for the balances test build. Other
callers of the reusable workflow are unaffected.

.github/workflows/balances_test.yml

@@ -14,6 +14,7 @@ jobs:
       upload-name: develop-apk
       run-tests: false
       build-debug-tests: true
+      free-disk-space: true
     secrets: inherit
 
   run-tests: