fix(ci): hardcode master branch for ct list-changed

Replace dynamic github.event.repository.default_branch with hardcoded
'master' to fix chart-testing list-changed command in both act and CI.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Aleksei Sviridkin <f@lex.la>

.github/workflows/pr-checks.yaml

@@ -60,7 +60,7 @@ jobs:
       - name: Run chart-testing (list-changed)
         id: list-changed
         run: |
-          changed=$(ct list-changed --target-branch ${{ github.event.repository.default_branch }} --config ct.yaml)
+          changed=$(ct list-changed --target-branch master --config ct.yaml)
           if [[ -n "$changed" ]]; then
             echo "changed=true" >> $GITHUB_OUTPUT
           fi