chart: skip existing releases in chart-release workflow

chart-release.yml fires on any charts/** push to main and ran
chart-releaser against the committed Chart.yaml version, failing with
422 already_exists whenever that version was already published. A tagged
release also publishes the chart from release.yml, which seds the
version from the tag, so the two workflows raced and this one lost.

Brings it to parity with the chart job in release.yml, which has carried
skip_existing since 1451682.

.github/workflows/chart-release.yml

@@ -29,5 +29,10 @@ jobs:
 
       - name: Run chart-releaser
         uses: helm/chart-releaser-action@v1.6.0
+        with:
+          # A charts/** push without a Chart.yaml version bump would otherwise
+          # fail trying to re-release the current version. Tagged releases also
+          # publish the chart from release.yml, so the two can race.
+          skip_existing: true
         env:
           CR_TOKEN: "${{ secrets.GITHUB_TOKEN }}"