Fix a typo to suppress updating rbconfig.rb during `make up`

.github/actions/setup/directories/action.yml

@@ -93,7 +93,7 @@ runs:
       working-directory: ${{ inputs.srcdir }}
       run: |
         touch config.status
-        touch rbconfig.time
+        touch .rbconfig.time
         sed -f tool/prereq.status template/Makefile.in common.mk > Makefile
         make up