runs test on main branch as well.

reasons:
- tests passed on PR branches do not mean they will pass on main branch,
  for e.g. alice changed A in branch-a, bob changed B in branch-b, both
  tests passed and no conflict, but after merged to main, a bug occurs.
- wanna fix the badge in main branch ;D

.github/workflows/ci.yml

@@ -1,6 +1,9 @@
 name: pgcli
 
 on:
+  push:
+    branches:
+      - main
   pull_request:
     paths-ignore:
       - '**.rst'