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/cc-wrapper.sh

···
 #! @shell@ -e
+path_backup=$PATH
+if [ -n "@coreutils@" ]; then
+  PATH="@coreutils@/bin:@gnugrep@/bin"
+fi
 
 if [ -n "$NIX_CC_WRAPPER_START_HOOK" ]; then
     source "$NIX_CC_WRAPPER_START_HOOK"
···
     source "$NIX_CC_WRAPPER_EXEC_HOOK"
 fi
 
+PATH=$path_backup
 exec @prog@ ${extraBefore[@]} "${params[@]}" "${extraAfter[@]}"

pkgs/build-support/cc-wrapper/default.nix

···
 , cc ? null, libc ? null, binutils ? null, coreutils ? null, shell ? stdenv.shell
 , zlib ? null, extraPackages ? [], extraBuildCommands ? ""
 , dyld ? null # TODO: should this be a setup-hook on dyld?
-, isGNU ? false, isClang ? cc.isClang or false
+, isGNU ? false, isClang ? cc.isClang or false, gnugrep ? null
 }:
 
 with stdenv.lib;
 
 assert nativeTools -> nativePrefix != "";
-assert !nativeTools -> cc != null && binutils != null && coreutils != null;
+assert !nativeTools ->
+  cc != null && binutils != null && coreutils != null && gnugrep != null;
 assert !nativeLibc -> libc != null;
 
 # For ghdl (the vhdl language provider to gcc) we need zlib in the wrapper.
···
 
   inherit cc shell;
   libc = if nativeLibc then null else libc;
-  binutils = if nativeTools then null else binutils;
-  # The wrapper scripts use 'cat', so we may need coreutils.
-  coreutils = if nativeTools then null else coreutils;
+  binutils = if nativeTools then "" else binutils;
+  # The wrapper scripts use 'cat' and 'grep', so we may need coreutils
+  # and gnugrep.
+  coreutils = if nativeTools then "" else coreutils;
+  gnugrep = if nativeTools then "" else gnugrep;
 
   passthru = { inherit nativeTools nativeLibc nativePrefix isGNU isClang; };
 

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

···
 #! @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"
···
     source "$NIX_GNAT_WRAPPER_EXEC_HOOK"
 fi
 
+PATH=$path_backup
 exec @prog@ ${extraBefore[@]} "${params[@]}" ${extraAfter[@]}

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

···
 #! @shell@ -e
+path_backup=$PATH
+if [ -n "@coreutils@" ]; then
+  PATH="@coreutils@/bin"
+fi
 
 if [ -n "$NIX_LD_WRAPPER_START_HOOK" ]; then
     source "$NIX_LD_WRAPPER_START_HOOK"
···
     source "$NIX_LD_WRAPPER_EXEC_HOOK"
 fi
 
+PATH=$path_backup
 exec @prog@ ${extraBefore[@]} "${params[@]}" ${extra[@]}

pkgs/stdenv/darwin/default.nix

···
       inherit stdenv shell;
       nativeTools = false;
       nativeLibc  = false;
-      inherit (pkgs) coreutils binutils;
+      inherit (pkgs) coreutils binutils gnugrep;
       inherit (pkgs.darwin) dyld;
       cc   = pkgs.llvmPackages.clang-unwrapped;
       libc = pkgs.darwin.Libsystem;