apple silicon (m1/m2) macOS default build of hello world fails.

I'm just started to play with Buck2 on mac os.
I have clang installed for Xcode but I did not install `llvm` `lld` with eg `brew install llvm` and did not set it up in zsh.
```
export LDFLAGS="-L/opt/homebrew/opt/llvm/lib"
export CPPFLAGS="-I/opt/homebrew/opt/llvm/include"
export PATH=/opt/homebrew/opt/llvm/bin:$PATH
```

if I run with the setup above:

building hello world example fails like this
```
-> % buck2 build //:main [master|●2…9]
File changed: root//.git/modules/prelude/index.lock
File changed: root//.git/index.lock
Action failed: root//:main (cxx_link_executable)
Local command returned non-zero exit code 1
Reproduce locally: `clang++ "-fuse-ld=lld" -o buck-out/v2/gen/root/213ed1b7ab869379/__main__/main @buck-out/v2/gen/root/213ed1b7ab869379/__main__/main.linker.argsfile`
stdout:
stderr:
clang: error: invalid linker name in argument '-fuse-ld=lld'
Build ID: 8491bb68-24f5-4d81-9696-3f1574e4ca0f
Jobs completed: 3. Time elapsed: 0.1s. Cache hits: 0%. Commands: 1 (cached: 0, remote: 0, local: 1)
```

After installing llvm and setting up the linker and path.
And more importantly, restarting buck2 with `buck2 kill` (took some time to figure out that changes in environment taking effect only after restart of the buck daemon)
it started to work like this:
```
-> % buck2 build //:main [master|●2…8]
Build ID: 49f3cd84-87cd-404e-80ff-7df9536143c8
Jobs completed: 53. Time elapsed: 2.4s. Cache hits: 0%. Commands: 5 (cached: 0, remote: 0, local: 5)
BUILD SUCCEEDED
```

I have in general 2 questions:

1. If the default toolchain is doing some kind of auto-discovery Why there is no check that the default settings are working (eg CMAKE's compiler diagnostic)?

2. to achieve hermetic builds I was expecting that bulk2 would install in a sandbox a toolchain which is matching with my default platform. Are there examples of how to create hermetic builds with sandboxes where the compiler toolchain is populated during the build?