Replace --create-if-missing to --update-only

`--update-only` is disabled by default, whereas `--create-if-missing` should've been enabled by default but was not.

.github/actions/post-comment/action.yml

@@ -9,12 +9,10 @@ inputs:
   body:
     description: "Full comment markdown."
     required: true
-  create-if-missing:
-    description: >
-      Create a comment if no matching marker exists.
-      Set to 'false' to update existing comments only.
+  update-only:
+    description: Do nothing if no comment with the specified marker exists.
     required: false
-    default: "true"
+    default: "false"
   issue-id:
     description: >
       The issue or pull request number to post/update the comment on.
@@ -31,13 +29,13 @@ runs:
       env:
         BODY: ${{ inputs.body }}
         MARKER: ${{ inputs.marker }}
-        CREATE_IF_MISSING: ${{ (inputs.create-if-missing == 'true' && '--create-if-missing') || '' }}
+        UPDATE_ONLY: ${{ (inputs.update-only == 'true' && '--update-only') || '' }}
         GH_TOKEN: ${{ inputs.token }}
         ISSUE_ID: ${{ inputs.issue-id || github.event.pull_request.number }}
       run: |
         npx tsx ./pr-checks/post-comment.ts \
           --body "$BODY" \
           --marker "$MARKER" \
-          $CREATE_IF_MISSING \
+          $UPDATE_ONLY \
           --issue-id "$ISSUE_ID" \
           --repository "$GITHUB_REPOSITORY"