Is EXECUTE AS OWNER in msdb a security risk?
In and of itself, no, you are probably not adding a security risk.  As long as no one but a sysadmin can alter the procedure and as long as the ONLY thing the procedure can do is start a job I don't see a problem.

The risks come in with the fact that MSDB is (and is supposed to be) marked as TRUSTWORTHY.  That means that by using EXECUTE AS OWNER you are able to impersonate the SERVER level permissions.  In this case since it's sa that means you could create a procedure to grant another user sysadmin.

That does mean you do want to limit permissions to MSDB though.