apple-clang: add correct path to compiler wrappers (#21662)

Follow-up to #17110

### Before
```bash
CC=/Users/Adam/spack/lib/spack/env/clang/clang; export CC
SPACK_CC=/usr/bin/clang; export SPACK_CC
PATH=...:/Users/Adam/spack/lib/spack/env/apple-clang:/Users/Adam/spack/lib/spack/env/case-insensitive:/Users/Adam/spack/lib/spack/env:...; export PATH
```

### After
```bash
CC=/Users/Adam/spack/lib/spack/env/clang/clang; export CC
SPACK_CC=/usr/bin/clang; export SPACK_CC
PATH=...:/Users/Adam/spack/lib/spack/env/clang:/Users/Adam/spack/lib/spack/env/case-insensitive:/Users/Adam/spack/lib/spack/env:...; export PATH
```

`CC` and `SPACK_CC` were being set correctly, but `PATH` was using the name of the compiler `apple-clang` instead of `clang`. For most packages, since `CC` was set correctly, nothing broke. But for packages using `Makefiles` that set `CC` based on `which clang`, it was using the system compilers instead of the compiler wrappers. Discovered when working on `py-xgboost@0.90`.

An alternative fix would be to copy the symlinks in `env/clang` to `env/apple-clang`. Let me know if you think there's a better way to do this, or to test this.

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