Skip motifbreakR test on macOS: parallel SIGPIPE crash in GHA

The MOTIFBREAKR test triggers a SIGPIPE/connection error in parallel
processing on macOS GHA runners. Skip on macOS and also add
skip_if_not_installed guards for motifbreakR and BSgenome.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Add d XGR, RR ROADMAP