The custom subcommand test fixture used to replace the environment
with PATH prepended with the directory of the subcommand.
This caused the tests to fail on darwin with auto-allocate-uids
enabled, as the dynamic users aren't added to the user database
inside the sandbox, as opposed to linux.
Other environments were unaffected because the build user is a real user
with a database entry and HOME set.
Update the environment instead, also preserving hermetic env
created earlier by NixCommand constructor.
Change-Id: I7e59fd69ff13d1d395316d857b63a356e1648159
Nikodem Rabuliński
3815dd5e64