Right now our bootstrapFiles-selecting algorithm uses the
`loongson2f.nix` bootstrapFiles (which were not built by Hydra).
These bootstrapFiles don't work anymore. They were added in 2010 by
40405d03ac.
This commit causes mipsel-linux native builds to use the Hydra-built
bootstrap files from this PR instead:
https://github.com/NixOS/nixpkgs/pull/183487