hack: use PKG_CONFIG from "go env"

Current implementation overwrites PKG_CONFIG if not defined
and set it to pkg-config. When a build is invoked using xx in
our Dockerfile, it will set PKG_CONFIG to the right value in
go environments depending on the target architecture: https://github.com/tonistiigi/xx/blob/8015613cccc2d3b689bf8189ec3604c4e4c94c34/base/xx-go#L75-L78

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

hack/make.sh

@@ -26,7 +26,7 @@ set -o pipefail
 export DOCKER_PKG='github.com/docker/docker'
 export SCRIPTDIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 export MAKEDIR="$SCRIPTDIR/make"
-export PKG_CONFIG=${PKG_CONFIG:-pkg-config}
+export PKG_CONFIG=${PKG_CONFIG:-$(go env PKG_CONFIG)}
 
 echo