Delay requiring irb until runtime

Fixes #32410.
Remove debugging code again
Remove fails_with? from the formula instance
Move Compiler struct into CompilerSelector namespace
Replace CompilerQueue with predetermined priority lists
Make --cc override the compiler selector
Call reset in the superclass implementation of setup_build_environment
Update brew-test-bot for compiler selection changes
imagemagick: undisrecommend ghostscript
graphicsmagick: update 1.3.20 bottle.
imagemagick: update 6.8.9-7 bottle.