[Chapel Merge] Make slurm-srun `--mem` overridable

Branch: refs/heads/main
Revision: e8225fb
Author: ronawho
Link: Unavailable
Log Message:

Merge pull request #20022 from ronawho/optional-mem-srun

Make slurm-srun --mem overridable

[reviewed by @benharsh]

In #13667, we started throwing --mem=0 for the slurm-srun launcher to
get access to all memory on each node. This is still the default we
want, but a secondary use for --mem is to request the high memory
nodes (e.g. to select 512G nodes in a partition that has mixed 256/512G
ones.) Update the code to allow overriding with CHPL_LAUNCHER_MEM.

Modified Files:
M runtime/src/launch/slurm-srun/launch-slurm-srun.c

Compare: https://github.com/chapel-lang/chapel/compare/f8d6cb18d683...e8225fb4bf87