Revert "[alt-builder] Revert to stable gcc"

This reverts commit f8d537d7
3 jobs from master in 7 minutes and 2 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Deploy
passed #160381
deploy:alt-builder

00:05:33

passed #160292
deploy:repository/aptly

00:01:28

failed #160291
deploy:alt-builder

00:00:52