Make OMEGA_LOG_TASKS a runtime environment variable

[grnydawn]

This PR replaces OMEGA_LOG_TASKS as a compile-time CMake option with a runtime environment variable for log task selection.

This feature was requested for integration with E3SM. It also allows users to change the MPI tasks that generate log files without rebuilding the Omega executable.

Testing passed on Perlmutter.

Checklist

• Documentation:
  • User's Guide has been updated
  • Developer's Guide has been updated
• Linting
  • Pre-commit run locally
• Building
  • CMake build does not produce any new warnings from changes in this PR
• Testing
  • Which machines CTest unit tests have been run on and indicate that are all passing.

[mark-petersen]

Passes CTests on Frontier for CPU and GPU.

[sbrus89]

@andrewdnolan, if you have time to review/approve it would be nice to get this one merged.

[andrewdnolan]

Testing:

In order to test this in the most realistic setting possible, I cherry picked the one commit from this branch onto my coupled-driver branch and set up a case where the ROOTPE_OCN was no-zero. These are the commands I ran:

CASE_DIR=$PSCRATCH/E3SMv3/omega-logging-test

./create_newcase \
    -case ${CASE_DIR} \
    -mach pm-gpu \
    -res T62_oQU240 \
    --compset COMEGA-JRA1p4
    
    
pushd $CASE_DIR

./xmlchange DEBUG=true
./xmlchange ROOTPE_OCN=3

./case.setup
./case.build


salloc --nodes 2 --qos interactive --time 00:15:00 --gpus-per-node 4 --constraint gpu --account e3sm_g

./case.submit --no-batch

grep "^3: (seq_comm_setcomm)" /pscratch/sd/a/anolan/e3sm_scratch/pm-gpu/omega-logging-test/run/e3sm.log.55687647.260708-110043

Which returns:

3: (seq_comm_setcomm)  init ID ( 17 OCN             ) pelist   =     3     6     1 ( npes =     4) ( nthreads =  1)( suffix =)

And then there's a ocn.log... file with the output I'd expect!

cat /pscratch/sd/a/anolan/e3sm_scratch/pm-gpu/omega-logging-test/run/ocn.log.55687647.260708-110043 | wc -l
# 7

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@grnydawn Sorry this took so long to get around to, but this looks great!