slurmd: set SLURM_CONF in prolog/epilog environment

Set SLURM_CONF in default prolog/epilog environment instead
of only in spank prolog/epilog environment.

This change fixes a potential hang during spank prolog/epilog
execution due to the possibility of memory allocation after
fork(2) and before exec(2) when invoking slurmstepd spank
prolog|epilog.

This also has the benefit that SLURM commands used in prolog and epilog
scripts will use the correct slurm.conf file.