https://github.com/NixOS/nixpkgs/blob/ce944e8d76f74336dbf6725c87ca2539b0653c30/pkgs/build-support/rust/lib/default.nix#L58-L82
This doesn't look quite right to me...
The build hook used in my builds contains this:
"CC_X86_64_UNKNOWN_LINUX\_MUSL=/nix/store/wrhxx96lh48nsv9a6p38b3x4r2fskwwm-gcc-wrapper-14.3.0/bin/cc" \
"CXX_X86_64_UNKNOWN_LINUX\_MUSL=/nix/store/wrhxx96lh48nsv9a6p38b3x4r2fskwwm-gcc-wrapper-14.3.0/bin/c++" \
"CARGO_TARGET_X86_64_UNKNOWN_LINUX_MUSL\_LINKER=/nix/store/wrhxx96lh48nsv9a6p38b3x4r2fskwwm-gcc-wrapper-14.3.0/bin/cc" \
"CARGO_BUILD_TARGET=x86\_64-unknown-linux-musl" \
"HOST\_CC=/nix/store/wrhxx96lh48nsv9a6p38b3x4r2fskwwm-gcc-wrapper-14.3.0/bin/cc" \
"HOST\_CXX=/nix/store/wrhxx96lh48nsv9a6p38b3x4r2fskwwm-gcc-wrapper-14.3.0/bin/c++" \
"CC_X86_64_UNKNOWN_LINUX\_MUSL=/nix/store/wrhxx96lh48nsv9a6p38b3x4r2fskwwm-gcc-wrapper-14.3.0/bin/cc" \
"CXX_X86_64_UNKNOWN_LINUX\_MUSL=/nix/store/wrhxx96lh48nsv9a6p38b3x4r2fskwwm-gcc-wrapper-14.3.0/bin/c++" \
"CARGO_TARGET_X86_64_UNKNOWN_LINUX_MUSL\_LINKER=/nix/store/wrhxx96lh48nsv9a6p38b3x4r2fskwwm-gcc-wrapper-14.3.0/bin/cc" \
but my platform definition is
nixpkgs.buildPlatform = (inputs.nixpkgs.lib.systems.elaborate "x86_64-unknown-linux-musl");
nixpkgs.hostPlatform =
inputs.nixpkgs.lib.recursiveUpdate
(inputs.nixpkgs.lib.systems.elaborate "x86_64-unknown-linux-musl")
{
useLLVM = true;
linker = "lld";
config = "x86_64-unknown-linux-musl";
};
So i'd have expected lld and clang in there | 
Grimmauld (migrated to @grimmauld:m.grimmauld.de)