nixos/rust-motd: allow ordering sections by `priority`

Closes #234802

The problem here is that with e.g.

    {
      uptime.prefix = "Up";
      banner.command = "hostname | figlet -f slant";
    }

`banner` still appears before `uptime` in the final motd text because
Nix sorts attribute names alphabetically internally.

To work around this without breaking compatibility or losing the
property to override individual sections in other modules - e.g.

    {
      banner.color = mkForce "blue";
    }

I decided to introduce an option `priority` here, similar to the
priority field for `nginx`[1] and with the same semantics (i.e. higher
value means lower priority).

Internally a bunch of env vars are generated, i.e. `env0` to `envN` for
`N` sections with each of them containing a declaration for the TOML,
i.e. `env0` contains `{ uptime.prefix = "Up"; }` and `env1` contains
`{ banner.command = "hostname | figlet -f slant"; }` if `uptime.priority`
is set to a value below 1000.

In this order, the declarations are concatenated together by `jq(1)`
which doesn't sort keys alphabetically which results in a JSON
representation with `uptime` before `banner`. This is finally piped to
`json2toml` which converts this into TOML for rust-motd.

[1] https://nixos.org/manual/nixos/unstable/options#opt-services.nginx.virtualHosts._name_.locations._name_.priority

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