[mgr/dashboard] build fails if python3-virtualenv-xx is not installed
In particular it fails late, for reasons that are hard to diagnose.
It would probably fail earlier, and be easier to diagnose, if .../src/tools/setup-virtualenv.sh used `set -e`
Found while trying to build on rhel9-beta which currently does not have python3-virtualenv-xx available, e.g. in its Code Ready Builder repo or the BaseOS repo.
(N.B. python3-virtualenv-xx and its dependencies are built in brew for rhel9, but at the time of this writing aren't in any package repos for rhel9)