This PR contains 3 changes:

 - Bump GCC in Guix from 12.3.0 to 12.4.0. A patch was sent upstream, https://lists.gnu.org/archive/html/guix-patches/2024-06/msg01025.html, but has not landed.
 - Consolidate all build environments back to using a GCC 12 toolchain. After
   build: LLD based macOS toolchain #21778, the macOS environment is no-longer pinned to 11 (12 would otherwise cause issues building cctools). So, instead of requiring all builders to compile an additional GCC toolchain, use 12.
 - Use GCC 12 to compile winpthreads. Currently, GCC 11 is used; which became apparent in
   guix: bump time-machine to efc26826400762207cde9f23802cfe75a737963c #30452 (comment).