ci: prevent incorrect pattern matching for skipping ci command

Mergify matches `body-raw` as regular expression, but that makes it
difficult (if not impossible) to match the `[` in a string.

Fixes: #4751
Signed-off-by: Niels de Vos <ndevos@ibm.com>

.mergify.yml

@@ -469,7 +469,8 @@ pull_request_rules:
   # the PR as Draft so that CI jobs do not automatically run anymore.
   - name: detect [skip ci] in the PR description
     conditions:
-      - "body-raw~=[skip ci]"
+      # this should be \[skip\W+ci\], but Mergify does not accept that
+      - "body-raw~=skip ci"
     actions:
       edit:
         draft: true