pkg-config: prepend added flags

The go toolchain calls pkg-config using "--" to separate flags and
packages. For example:

pkg-config --cflags -- zlib

This breaks if addFlags are appended. This commit therefore prepends the
addFlags to the command.

pkgs/build-support/pkg-config-wrapper/pkg-config-wrapper.sh

@@ -12,7 +12,7 @@ if [ -z "${NIX_PKG_CONFIG_WRAPPER_FLAGS_SET_@suffixSalt@:-}" ]; then
     source @out@/nix-support/add-flags.sh
 fi
 
-set -- "$@" @addFlags@
+set -- @addFlags@ "$@"
 
 if (( ${#role_suffixes[@]} > 0 )); then
     # replace env var with nix-modified one