🚦 Hard-coding the "before" chart URL in the PR diff GitHub action

While this may not be strictly necessary, it's the correct resolution
enough of the time that the clarity of explicitly stating the "before"
is, I think, worth it.

.github/workflows/pr_diff.yaml

@@ -1,4 +1,5 @@
 name: PR Diff for Helm chart
+
 on:
   pull_request_target:
 
@@ -37,7 +38,7 @@ jobs:
             --set s3.bucket=abc
             --set s3.encrypt=abc
           )
-          helm template --debug ${OPTIONS[@]} --output-dir before ${{ github.server_url }}/${{ github.repository }}/archive/refs/heads/${{ github.base_ref }}.tar.gz
+          helm template --debug ${OPTIONS[@]} --output-dir before https://github.com/twuni/docker-registry.helm/archive/refs/heads/main.tar.gz
           helm template --debug ${OPTIONS[@]} --output-dir after .
           # https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions#multiline-strings
           echo 'HELM_DIFF<<EOF' >> $GITHUB_ENV