buildGoModule: access `ldflags` and `tags` `__structuredAttrs`-agnostically (#360037)

pkgs/build-support/go/module.nix

···
         local cmd="$1" dir="$2"
 
         declare -a flags
-        flags+=(''${tags:+-tags=''${tags// /,}})
-        flags+=(''${ldflags:+-ldflags="$ldflags"})
+        flags+=(''${tags:+-tags=$(concatStringsSep "," tags)})
+        flags+=(''${ldflags:+-ldflags="''${ldflags[*]}"})
         flags+=("-p" "$NIX_BUILD_CORES")
         if (( "''${NIX_DEBUG:-0}" >= 1 )); then
           flags+=(-x)
···
     '' + lib.optionalString (stdenv.hostPlatform != stdenv.buildPlatform) ''
       # normalize cross-compiled builds w.r.t. native builds
       (
-        dir=$GOPATH/bin/${go.GOOS}_${go.GOARCH}
+        dir=$GOPATH/bin/''${GOOS}_''${GOARCH}
         if [[ -n "$(shopt -s nullglob; echo $dir/*)" ]]; then
           mv $dir/* $dir/..
         fi