Add common CI script helpers

[rbberger]

So far we've encouraged the use of

salloc <SCHEDULER_PARAMS> / flux alloc <SCHEDULER_PARAMS>
source .gitlab/kessel.sh
kessel run --env <ENV-NAME>

for Spack-based CI pipelines. This PR adds helpers that make a simpler approach possible:

source .gitlab/alloc.sh <ENV-name>
kessel run

Note that how alloc.sh is implemented is project specific, but the helpers in this PR make it easier to write it.

[AndresYague]

Save from the above minor change, I think this looks good and useful.