hack/module.sh sets MOD_DIR for module tasks

This is to allow tasks which need to be executed in a module-specific
context to detect that they are being invoked appropriately.

hack/module.sh

@@ -51,6 +51,7 @@ function with_modules() {
   for mod_file in $(find . -maxdepth 4 -name go.mod | sort); do
     mod_dir="$(dirname "${mod_file}")"
     (
+      export MOD_DIR="$(pwd)"
       echo "=> " && \
       echo "   cd ${mod_dir} && ${cmd}" && \
       cd "${mod_dir}" && ${cmd}