Only use charm built with gcc on CI

[knelli2]

On both local and Wheeler, I've been using charm built with gcc but SpECTRE built with clang and I've had no issues

Proposed changes

Separated from #4673 so we don't interrupt CI when we update the docker container.

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

[wthrowe]

As an additional data point, I've also been using one charm build with different compilers without a problem on my dev machine.

[nilsvu]

The two clang Release builds took a very long time because their cache was invalidated by this change. Not really sure what to do about that. I restarted them for now.

[nilsvu]

They passed now, so I'm merging this and hoping for caching to kick in. The one clang release build took a scary 5.5h though, so we may have to do something about build times soon..