fix(ci): grant the label job pull-requests write (release-1.12.0) (#14591)

fix(ci): grant the label job pull-requests write

Every "Label PR" run has failed since #14540 -- 67 successes and no failures
before it, 16 failures after (the successes since are runs where the job's `if:`
skips it, e.g. merge_group and bot PRs):

    POST /repos/langflow-ai/langflow/issues/14588/labels
    403 Resource not accessible by integration

#14540 added a `permissions:` block to this workflow. Before that there was
none, so it inherited the repository default, which includes pull-requests
write. Labelling a *pull request* needs that scope: the `issues` permission only
covers real issues even though the REST path is `/issues/{n}/labels`. GitHub
says so in the response itself:

    x-accepted-github-permissions: issues=write; pull_requests=write

Also unblocks Namchee/conventional-pr in the same workflow, which cannot post
its report under a read-only pull-requests scope.

(cherry picked from commit 9454ac40d4b5539e6b704cb949649913ed9e8146)