check-meta: Add isHydraChannel

This is needed in order to mark a certain derivation containing a Nix
expression tarball to Hydra so that it is recognised as a channel.

When I first got an evaluation error due to using this meta attribute, I
was under the impression that nobody outside of Vuizvui[1] is using this
feature and that we don't have any occurrence of isHydraChannel in
Nixpkgs.

However, when working around[2] the issue I assumed that it's not
something that should be included in Nixpkgs because we're not using it
there.

It turned out that my assumption was wrong and we *do* use the attribute
in Nixpkgs, namely via releaseTools.channel, which is similar to what
we're doing in Vuizvui.

Since we already include a bunch of undocumented attributes in
metaTypes, it only makes sense to add isHydraChannel as well since it's
actually documented in the Hydra documentation[3].

[1]: https://github.com/openlab-aux/vuizvui
[2]: https://github.com/openlab-aux/vuizvui/commit/e0685e81b3fdc43a272f0
[3]: https://github.com/NixOS/hydra/blob/53335323ae79ca1a42643f58e520b376898ce641/doc/manual/src/jobs.md#meta-fields

Signed-off-by: aszlig <aszlig@nix.build>

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