Add permission to post the terraform plan in a comment (#2733)

After [restricting the id-token
permission](https://github.com/bazelbuild/continuous-integration/pull/2679/changes)
to the "detect-changes" steponly, it was stripped off any other general
permissions like: "pull-requests: write" (which was defined at the top
of the workflow)

We need to re-add it to this step to be able to post a comment with the
plan