Fix Slurm handling of comma-separated env vars

[al-rigazzi]

This is a minimal-modification PR. Whenever an env var is found to contain a comma, we set it locally to the srun process and add the name of then env var (not the value!) to the list of exported env vars. Its value is then correctly passed by srun to the launched application. This does not change anything in how other env variables are exported/formed/formatted and the result is still human-readable.

Basically, instead of trying to run the unsuccessful command:

srun --export SSKEYIN=name_0,name_1,OTHER_VAR=value python script.py

we run

SSKEYIN=name_0,name_1 srun --export SSKEYIN,OTHER_VAR=value python script.py

[Spartee]

LGTM, glad to have this fix in.