session-worker: set PWD for user session

Merged Ray Strode requested to merge wip/pwd-fix into master

It's possible for the current working directory to be a symlink. Some programs, and notably, the get_current_dir_name() libc api use the PWD environment variable to know the symlink being used (so they don't use the canonical name that the symlink points to)

This commit makes sure we set PWD in that case, to allow those programs to show the right path.

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