setup.sh: add HOST_PATH

HOST_PATH contains the path of the host package. This will include the
packages listed in buildInputs & depsHostHost. Use this to find
runtime commands that the host needs.

For instance to find the runtime version of perl,

$ PATH="$HOST_PATH" command -v perl
/nix/store/...-perl-5.28.0-aarch64-unknown-linux-android/bin/perl

This path should not be executed directly (it will break for cross
compilation). Only use it to find the location of executables that
will be run by your host system. Your build tools will, as always, be
available on the default PATH.

pkgs/stdenv/generic/setup.sh

@@ -504,6 +504,10 @@ activatePackage() {
         addToSearchPath _PATH "$pkg/bin"
     fi
 
+    if [[ "$hostOffset" -eq 0 && -d "$pkg/bin" ]]; then
+        addToSearchPath HOST_PATH "$pkg/bin"
+    fi
+
     if [[ -f "$pkg/nix-support/setup-hook" ]]; then
         local oldOpts="$(shopt -po nounset)"
         set +u