nixos/gitlab: postgresql: Make PSQL a function, run as superUser
nixos/gitlab: postgresql: Make PSQL a function, run as superUser

A function is more appropriate for this use. See
http://mywiki.wooledge.org/BashFAQ/050 for reference.

Also, we don't need to run the service as root: since we essentially
run all commands as `services.postgresql.superUser` anyway, the whole
service can just run as that user instead.

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