ci: pin upload-artifact to v3

v4's GHES check refuses to run on any non-github.com server, so the
snap artifact upload always failed on gitea; v3 uses the artifact API
gitea implements.

.github/workflows/ci.yml

@@ -114,7 +114,9 @@ jobs:
           export PATH="/usr/libexec/snapcraft:$HOME/.cargo/bin:$PATH"
           snapcraft pack --destructive-mode
       - name: Upload snap artifact
-        uses: actions/upload-artifact@v4
+        # v4 refuses to run outside github.com (GHESNotSupportedError); Gitea
+        # implements the artifact API used by v3, so v3 is the supported choice.
+        uses: actions/upload-artifact@v3
         with:
           name: snap
           path: ./*.snap