ci: fix Mac builds broken by llvm@20 pin in #47

Summary:
The `llvm@20` pin that landed in facebook/rebalancer#47 breaks Mac builds. Homebrew's folly bottles are compiled against LLVM 22's libc++; linking against LLVM 20 produces `ld: symbol(s) not found for architecture arm64` on libfolly.

The correct fix (which was developed but didn't make it into the facebook/rebalancer#47 squash merge) is to:
1. Revert the `llvm@20` pin back to `brew install llvm` (LLVM 22) in `getdeps_mac.yml`, `before_all_macos.sh`, and `wheels.yml`
2. Remove `fmt` from `[homebrew]` in `manifests/fmt` so getdeps builds fmt 12.1.0 from source as a static library — no `-DFMT_SHARED`, so `fmt::format` stays header-inline and visible under LLVM 22's libc++

This matches Linux behavior (getdeps already builds fmt from source there).

X-link: facebook/rebalancer#48

Reviewed By: yangneu2015

Differential Revision: D110918926

Pulled By: r-barnes

fbshipit-source-id: fd15b2f83f14bd8c6077a6ecf210eeebf106e459

Author: r-barnes

build/fbcode_builder/manifests/fmt

@@ -20,8 +20,5 @@ FMT_DOC = OFF
 [cmake.defines.feature_shared_libs=on]
 BUILD_SHARED_LIBS = ON
 
-[homebrew]
-fmt
-
 [rpms.distro=fedora]
 fmt-devel