Add GCC 11 to CI

[ripatel-fd]

Lots of operators use GCC 11 to compile Firedancer because it's the Ubuntu 22.04 default.
The FD dev team doesn't use this version at all.
Recently, a compiler bug on GCC 11 slipped by in a release that prevented a build: #3753

Therefore GCC 11 should be added at least to nightly CI to prevent further regressions.