Actually test nix-env with a remote store

The `remote-store` test loads the `user-env` one to test nix-env when
using the daemon, but actually does it incorrectly because every test
starts (in `common.sh`) by resetting the value of `NIX_REMOTE`, meaning
that the `user-env` test will never use the daemon.

Fix this by setting `NIX_REMOTE_` before sourcing `user-env.sh` in the
`remote-store` test, so that `NIX_REMOTE` is correctly set inside the
test