cc-wrapper: fix on darwin

The ld-wrapper.sh script calls `readlink` in some circumstances. We need
to ensure that this is the `readlink` from the `coreutils` package so
that flag support is as expected.

This is accomplished by explicitly setting PATH at the top of each shell
script.

Without doing this, the following happens with a trivial `main.c`:

```
nix-env -f "<nixpkgs>" -iA pkgs.clang
$ clang main.c -L /nix/../nix/store/2ankvagznq062x1gifpxwkk7fp3xwy63-xnu-2422.115.4/Library -o a.out
readlink: illegal option -- f
usage: readlink [-n] [file ...]
```

The key element is the `..` in the path supplied to the linker via a
`-L` flag. With this patch, the above invocation works correctly on
darwin, whose native `/usr/bin/readlink` does not support the `-f` flag.

The explicit path also ensures that the `grep` called by `cc-wrapper.sh`
is the one from Nix.

Fixes #6447

pkgs/build-support/cc-wrapper/gnat-wrapper.sh

@@ -1,4 +1,8 @@
 #! @shell@ -e
+path_backup=$PATH
+if [ -n "@coreutils@" ]; then
+  PATH="@coreutils@/bin"
+fi
 
 if [ -n "$NIX_GNAT_WRAPPER_START_HOOK" ]; then
     source "$NIX_GNAT_WRAPPER_START_HOOK"
@@ -100,4 +104,5 @@ if [ -n "$NIX_GNAT_WRAPPER_EXEC_HOOK" ]; then
     source "$NIX_GNAT_WRAPPER_EXEC_HOOK"
 fi
 
+PATH=$path_backup
 exec @prog@ ${extraBefore[@]} "${params[@]}" ${extraAfter[@]}