Bump version of unity-builder

[mfbrantner]

The action unity-builder currently uses version 2. It can now be bumped to version 3.

Upgrading unity-builder requires a slight change to the build script, since it searches for a certain string in the log.

Note: Version 3 of unity-test-runner is also available, but there appears to be an issue with the edit-mode tests running indefinitely, so it should stay on v2 for the time being.

[FlorianGlawogger]

Since the last builds with the v3 version did not work correctly, the PR #453 got reverted again via #458.

[FlorianGlawogger]

According to game-ci/unity-builder#596 v4 of the UnityBuilder should handle the errors the same way as v2 did, maybe we can skip v3 and immediately go to v4?

[FlorianGlawogger]

unity-builder v4 has been merged into develop with #457