Fix naming of the source tarball (#6068)

This refactoring from #5766 accidentally broke the tag name calculation
for the `build-src-tarball.sh` script so this fixes it by copying over
the same logic from `build-tarballs.sh`.

ci/build-src-tarball.sh

@@ -4,8 +4,8 @@ set -ex
 
 # Determine the name of the tarball
 tag=dev
-if [[ $GITHUB_REF == refs/tags/v* ]]; then
-  tag=${GITHUB_REF#refs/tags/}
+if [[ $GITHUB_REF == refs/heads/release-* ]]; then
+  tag=v${GITHUB_REF:19}
 fi
 pkgname=wasmtime-$tag-src