Use a tag in create-job-server-pr.yml

`just update-interactive-templates` in job-server currently requires a
repository tag to be specified.

So what was happening prior to this commit:

1. `just update-interactive-templates` in the job-server repository,
   runs with the variable `GITHUB_SHA` passed to it.
1. `update-interactive-templates` uses this value when
   updating the requirements.
   The `GITHUB_SHA` is a commit hash and does not correspond to a tag,
   `update-interactive-templates` then rewrites the requirements file to
   point to a URL of the format:
   `https://github.com/opensafely-core/interactive-templates/archive/refs/tags/<commit_hash>.zip'
1. The requirements are rebuilt with `pip-tools`. This fails as the
   updated URL for `interactive-templates` does not exist. We're looking for a literal
   tag that *is* a commit hash, and our actual tags are release versions
   specified as `v2024.…`

Perhaps the intent here was originally to match a commit SHA to a tag?
And we could change `update-interactive-templates` to find the tag for a
given commit. But, by sticking with a tag — how
`update-interactive-templates` works — instead of using a commit SHA, we also
solve the following issue when running this workflow via
`workflow_dispatch`.

Let's say:

1. We made a new release with a specific version.
2. The release fails due to some error in the workflow.
3. We fix up the workflow and merge that fix.
4. We then try and re-release the version, but if
   `update-interactive-templates` is looking for a commit SHA to match
   to a tag, that commit SHA when the workflow dispatch runs won't match
   the release tag, so will still fail.

Instead, let's simplify things: in all cases that this workflow is run,
we pick the most recent tag, and if there's any change to the
requirements file, then create the PR in job-server.

.github/workflows/create-job-server-pr.yml

@@ -27,7 +27,8 @@ jobs:
           python-version: "3.12"
 
       - name: Update requirement to latest
-        run: just update-interactive-templates "$GITHUB_SHA"
+        run: |
+          just update-interactive-templates "$(git describe --tags)"
 
       - name: Create a Pull Request if there are any changes
         id: create_pr