Fix CI directory exist in release archive

Contrary to the gitattributes(5) manpage, directories are required to be excluded from archive with `path/ export-ignore` instead of `path/**`.

Refer-to: git ignoring .gitattributes pattern - Stack Overflow <https://stackoverflow.com/a/5525195/6219078>
Signed-off-by: 林博仁(Buo-ren Lin) <Buo.Ren.Lin@gmail.com>

Author: brlin-tw

.gitattributes

@@ -13,4 +13,4 @@
 
 # Exclude development assets from release archive
 /.* export-ignore
-/continuous-integration/** export-ignore
+/continuous-integration/ export-ignore