nixosTests.postgresql-wal-receiver is broken

[tfc]

When running the postgresql-wal-receiver test, the recovery-done step times out:

$ nix-build nixos/tests/postgresql-wal-receiver.nix
...
machine: running command: test -e /var/db/postgresql/test/recovery.done
machine: exit status 1
(0.00 seconds)
(902.90 seconds)
error: action timed out after -1 seconds at /nix/store/86hrdp84dfdpdp3iay64vn34srrbh4k0-nixos-test-driver/lib/perl5/site_perl/Machine.pm line 247, <__ANONIO__> line 14648.
(984.92 seconds)
action timed out after -1 seconds at /nix/store/86hrdp84dfdpdp3iay64vn34srrbh4k0-nixos-test-driver/lib/perl5/site_perl/Machine.pm line 247, <__ANONIO__> line 14648.
cleaning up
killing machine (pid 6)
(0.00 seconds)
vde_switch: EOF on stdin, cleaning up and exiting
vde_switch: Could not remove ctl dir '/build/vde1.ctl': Directory not empty
builder for '/nix/store/zypb8rh7w5dcia0yklb6kwx7qny4pw95-vm-test-run-postgresql-wal-receiver-postgresql_12.drv' failed with exit code 255
error: build of '/nix/store/zypb8rh7w5dcia0yklb6kwx7qny4pw95-vm-test-run-postgresql-wal-receiver-postgresql_12.drv' failed

/cc @pacien @danbst (commit history)