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.