nixUnstable was removed in 2b4e18f3d4
but is still referred to from a variety of places, including Hydra's
cross-trunk jobset:
```
in job ‘armv7l-hf.nixUnstable.aarch64-linux’:
error:
… while evaluating a branch condition
at /nix/store/m549zq6lcl10d1jqcdgcdaxxj9xdhxfv-source/lib/customisation.nix:479:5:
478| in
479| if drv == null then null else deepSeq drv' drv';
| ^
480|
… in the left operand of the update (//) operator
at /nix/store/m549zq6lcl10d1jqcdgcdaxxj9xdhxfv-source/lib/meta.nix:43:9:
42| addMetaAttrs = newAttrs: drv:
43| drv // { meta = (drv.meta or {}) // newAttrs; };
| ^
44|
(stack trace truncated; use '--show-trace' to show the full, detailed trace)
error: nixUnstable has been removed. For bleeding edge (Nix
master, roughly weekly updated) use nixVersions.git, otherwise
use nixVersions.latest.
```
dd5617fef7