Plugins reading in their own config files rely on the SLURM_CONF
environment variable pointing to the appropriate directory,
otherwise they will fall back to the build in sysconfdir path.

Set the environment variable early enough so that the -f flag
operates correctly, but not before conf->conffile has definitely
been set. Remove the setenv call that happens before the first
slurmstepd is fork()'d as it is now redundant.

Bug 4774.