ci: more granular cloud run labels

[gforsyth]

We added ci-run-cloud in #8280 to simplify running credentialed cloud backends on PRs -- modulo that we can't get them to stop cancelling each other, this has been a definite improvement.

We often have PRs that are targeting just a part of the Bigquery or Snowflake backend -- esp. in the case of snowflake-only changes, we would benefit from a ci-run-snowflake, because the bigquery tests can drag quite a bit and we don't want to sit around and wait for unrelated tests to pass.