Use boehmgc package from nixpkgs in shell.nix

[straight-shoota]

This is largely a simplification. I don't think we need to build our own derivation of the GC when there's already a generic one in nixpkgs. With overrides we get almost the same configuration as our custom build.
The custom build was previously necessary because we required unreleased patches.
That's gone now and we should be fine with any recent upstream version.

Most significant changes as far as I can see are some compile flags are gone:

• --disable-debug: Not sure if there's any particular reason for this.
• --disable-dependency-tracking: There seems to be a reason to use this for building unified binaries on macOS. https://github.com/ivmai/bdwgc/blob/b1fe06200d716cd63d3ec321654fb4d03d610240/docs/platforms/README.darwin#L9
  I don't think this is necessary for a development environment, so it's probably fine to drop it.
• --disabled-shared should not be necessary. We could set enableStatic instead but I don't think that's necessary for a development environment.

[bcardiff]

Something I liked about the previous state is that jumping to a patched gc version was easier :-)

[mamantoha]

Hi @straight-shoota . Could this change be related to the issue with crystal-lang/install-crystal GitHub Action that causes an error with Crystal Nightly? Check this link for an example.

[straight-shoota]

No, that's completely unrelated. The installer issue was caused by crystal-lang/distribution-scripts#300 and is already fixed (#14594). Unfortunately, today's nightly build is broken for yet another reason, but should be fixed shortly by #14595.