ci: retry snap store upload on transient connection drops (#38461)

Add a retry to snapstore. Claude can't seem to figure out the reason why
so many nightly uploads fail and the only logical conclusion is it's a
server-side issue at Canonical, so retries might help.

Example:
https://github.com/go-gitea/gitea/actions/runs/29395231850/job/87287126446

.github/workflows/release-nightly-snapcraft.yml

@@ -27,9 +27,14 @@ jobs:
           fetch-depth: 0
       - uses: snapcore/action-build@3bdaa03e1ba6bf59a65f84a751d943d549a54e79 # v1.3.0
         id: build
-      - uses: snapcore/action-publish@214b86e5ca036ead1668c79afb81e550e6c54d40 # v1.2.0
+      # retry snapcraft uploads which can be flaky
+      - name: Publish snap
+        run: |
+          for attempt in 1 2 3 4 5; do
+            snapcraft upload "${{ steps.build.outputs.snap }}" --release latest/edge && exit 0
+            echo "::warning::snap upload attempt $attempt failed, retrying in 15s"
+            sleep 15
+          done
+          exit 1
         env:
           SNAPCRAFT_STORE_CREDENTIALS: ${{ secrets.SNAPCRAFT_STORE_CREDENTIALS }}
-        with:
-          snap: ${{ steps.build.outputs.snap }}
-          release: latest/edge