[CI] Fix SemVer check base commit (#5361)

After seeing some cases of reported changes that did not happen by the
merge request proposer (like
https://github.com/paritytech/polkadot-sdk/pull/5339), it became clear
that [this](https://github.com/orgs/community/discussions/59677) is
probably the issue.
The base commit of the SemVer check CI is currently using the *latest*
master commit, instead of the master commit at the time when the MR was
created.

Trying to get the correct base commit now. For this to be debugged, i
have to wait until another MR is merged into master.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

.github/scripts/generate-prdoc.py

@@ -96,6 +96,7 @@ def create_prdoc(pr, audience, title, description, patch, bump, force):
 	# write the parsed PR documentation back to the file
 	with open(path, "w") as f:
 		yaml.dump(prdoc, f)
+		print(f"PrDoc for PR {pr} written to {path}")
 
 def parse_args():
 	parser = argparse.ArgumentParser()

.github/workflows/check-semver.yml

@@ -12,7 +12,6 @@ concurrency:
 env:
   TOOLCHAIN: nightly-2024-06-01
 
-
 jobs:
   check-semver:
     runs-on: ubuntu-latest
@@ -20,14 +19,14 @@ jobs:
       image: docker.io/paritytech/ci-unified:bullseye-1.77.0-2024-04-10-v20240408
     steps:
       - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332 # v4.1.7
+        with:
+          fetch-depth: 2
 
       - name: extra git setup
-        env:
-          BASE: ${{ github.event.pull_request.base.sha }}
         run: |
           git config --global --add safe.directory '*'
-          git fetch --no-tags --no-recurse-submodules --depth=1 origin $BASE
-          git branch old $BASE
+
+          git branch old HEAD^1
 
       - name: Comment If Backport
         if: ${{ startsWith(github.event.pull_request.base.ref, 'stable') }}