This PR adds Hydra-generated bootstrap tarballs for mips64el-linux. I'll be following the script established in https://github.com/NixOS/nixpkgs/pull/151399, which I previously used in https://github.com/NixOS/nixpkgs/pull/168199.
Files came from [this](https://hydra.nixos.org/build/182757245) Hydra build, which used nixpkgs revision ef3fe254f3 to instantiate:
```
/nix/store/a2bvv663wjnyhq8m7v84aspsd3sgf9h6-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64.drv
```
and then built:
```
/nix/store/aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64
```
I downloaded these files from Hydra with the following commands:
```
STOREPATH=aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64
OPTIONS="--option binary-caches https://cache.nixos.org"
nix store add-file \
--name bootstrap-tools.tar.xz \
$(nix-store ${OPTIONS} -r /nix/store/${STOREPATH})/on-server/bootstrap-tools.tar.xz
nix store add-path \
--name busybox \
$(nix-store ${OPTIONS} -r /nix/store/${STOREPATH})/on-server/busybox
```
I then prefetched them into `/nix/store` with:
```
$ nix store prefetch-file --executable file:///nix/store/aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64/on-server/busybox
Downloaded 'file:///nix/store/aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64/on-server/busybox' to '/nix/store/ai30ss23914syz6j8m95arkwffbbx44k-busybox' (hash 'sha256-sTE58ofjqAqX3Xtq1g9wDxzIe6Vo//GHbicfqJoivDI=').
$ nix store prefetch-file file:///nix/store/aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64/on-server/bootstrap-tools.tar.xz
Downloaded 'file:///nix/store/aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64/on-server/bootstrap-tools.tar.xz' to '/nix/store/nr6zjrbwbxcxs6brf042zsyqllwbwj9v-bootstrap-tools.tar.xz' (hash 'sha256-tTgjeXpd2YgnfP4JvRuO0bXd2j8GqzBcd57JI3wH9x0=').
```
And started the bootstrap with the following command (the `--arg localSystem` is needed because #161159 has not merged):
```
nix build -f . -L hello --arg localSystem '(import ./lib).systems.examples.mips64el-linux-gnuabi64'
```
As @lovesegfault requested in #151399, here are the the `sha256sum`s of all the `on-server` components for extra verification:
```
$ sha256sum /nix/store/aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64/on-server/*
b53823797a5dd988277cfe09bd1b8ed1b5ddda3f06ab305c779ec9237c07f71d /nix/store/aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64/on-server/bootstrap-tools.tar.xz
598e05abb69b2c1a0db46585cd2131212077c0937ce2a665daf3811f059ae767 /nix/store/aw3dmsrh22831l83vi3q9apg9qi3x8ms-stdenv-bootstrap-tools-mips64el-unknown-linux-gnuabi64/on-server/busybox
```
73811b372e