buildGoModule: pre-initialize module cache (#61967)

For some Go projects a go.mod file is included but there are no listed
dependencies. When this is encountered the Go toolchain will not create
a cache folder for downloaded dependencies which causes buildGoModule to
fail.

An example of a project like this that is widely used is:
https://github.com/golang/protobuf

This commit adds a mkdir command to ensure that the directory always
exists so it can be copied and prevent the failure.

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

@@ -54,6 +54,7 @@ let
 
       export GOCACHE=$TMPDIR/go-cache
       export GOPATH="$TMPDIR/go"
+      mkdir -p "''${GOPATH}/pkg/mod/cache/download"
 
       runHook postConfigure
     '';