Fix update Dependabot PR workflow (#3065)

This fixes a small thing I missed in #3064.

Author: Mrtenz

.github/workflows/update-pull-request.yml

@@ -17,7 +17,7 @@ on:
 jobs:
   is-fork-pull-request:
     name: Determine whether this issue comment was on a pull request from a fork
-    if: ${{ github.event.issue.pull_request && startsWith(github.event.comment.body, '@metamaskbot update-pr') }}
+    if: ${{ github.event.issue.pull_request && startsWith(github.event.comment.body, '@metamaskbot update-pr') || inputs.dependabot == true }}
     runs-on: ubuntu-latest
     outputs:
       IS_FORK: ${{ steps.is-fork.outputs.IS_FORK }}