Can not set negative priorities with nix-env

[andrewhamon]

Describe the bug

I can set a positive priority:

nix-env --set-flag priority 100 python3-3.8.13
setting flag on 'python3-3.8.13'
...

But I can not set a negative priority:

nix-env --set-flag priority -100 python3-3.8.13
error: unknown flag '-100'
Try '/nix/store/c2655winvkf91w744rd6jdihjniiziap-nix-2.22.0/bin/nix-env --help' for more information.

If I try priority=, nix-env silently exits 0, but doesn't actually change the priority:

nix-env --set-flag priority=-100 python3-3.8.13
# does nothing

Steps To Reproduce

1. nix-env --set-flag priority -100 foo

Expected behavior

I should be able to set negative priorities.

nix-env --version output

nix-env (Nix) 2.22.0

Additional context

lib.hiPrio uses negative values, which tells me that negative values are valid. I can't find any documentation that indicates that negative values are not valid.

Priorities

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