Test flux 0.73.0 (when it appears) for fixes of two rare invocation issues

[rountree]

Version 0.71.0 of Flux does not parse spindle options correct in two cases. This case causes a segfault:

flux run \
    -o userrc=${FLUXRC} \
    --tasks-per-node=3 \
    --nodes=3 \
    python3 --version

The workaround is to add -o spindle.

This case causes an error:

flux run \
    -o userrc=${FLUXRC} \
    -o spindle \
    -o spindle.python-prefix=/usr/bin \
    --tasks-per-node=3 \
    --nodes=3 \
    /bin/true

The workaround is to remove -o spindle.

The underlying issue is how Flux was setting up option tree. The fix is described here. Hopefully that will be rolled into the 0.73.0 release.

This issue is a reminder to test these conditions once 0.73.0 shows up on local machines.

These issues are not at all critical and don't warrant downloading and building a one-off version of flux for testing.