fix running command as 'easybuild' user in generated Singularity definition file

[boegel]

fixes #3171

Switching to another user no longer works, and after consulting with the Singularity developers it seems this was actually working by accident in earlier versions of Singularity.

As an alternative, we run a set of commands as the easybuild user by using a here document.

The quoted 'EOF' is important to ensure that $LMOD_DIR does not get resolved too early, which leads to a confusing error like:

/update_lmod_system_cache_files -d /app/lmodcache -t /app/lmodcache/timestamp /app/modules/all
-bash: line 28: /update_lmod_system_cache_files: No such file or directory
FATAL:   failed to execute %post proc: exit status 1