goBuildPackage: Add -x to the go build command if NIX_DEBUG >= 1

The -x go option prints all intermediate commands used by the Go
compiler. For instance, this is pretty useful to debug Go LD_FLAGS
because the used linker command is printed.

pkgs/development/go-modules/generic/default.nix

···
       fi
     }
 
+    if (( "''${NIX_DEBUG:-0}" >= 1 )); then
+      buildFlagsArray+=(-x)
+    fi
+
     if [ ''${#buildFlagsArray[@]} -ne 0 ]; then
       declare -p buildFlagsArray > $TMPDIR/buildFlagsArray
     else