[CI] tag codec-msan job with ivas-linux-fast

Fyi, @pihlajakuja @norvell.

With the extra 5 and 10 ms decoder runs, the MSAN job in the MR pipeline can take a while on slow runners, e.g.: https://forge.3gpp.org/rep/ivas-codec-pc/ivas-codec/-/jobs/239527 - took almost 30 minutes.

The tag ensures that it is picked up by a fast runner.