brew-test-bot: check CompilerSelectionError twice.

It may be that even installing a new compiler doesn't fix things so
detect that case and skip the formula installation rather than erroring
out.
xctool: add 0.1.15 bottle.
Varnish 4.0.0
varnish: add 4.0.0 bottle.

Closes #28410.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>