This allows conveniently install a rust version with 'minimal' profile
for quick testing via the flake interface.
```
$ nix shell github:oxalica/rust-overlay#rust_1_76_0.minimal
```
This reverts commit cf102ae729, reversing
changes made to ffe47b9007.
Nix doesn't allow a dependent flake to have relative inputs.
```
error: cannot fetch input 'path:./systems.nix?lastModified=1&narHash=sh
a256-cyfZqnwhPNzdfQnmUUUVK5fon88TSm/0CwgE7Lf%2BLDU%3D' because it uses a relat
ive path
```
Closes#125
- `defaultOverlay` -> `overlays.default`
The old name is kept but with a warning.
- `defaultApp` is removed.
- `defaultPackage` -> `packages.default`
- Names in `packages` now use `_` instead of `-` for version separator.
Eg. `rust_1_49_0`, `rust-nightly_2022-06-29`.
- `rust{,-nightly,-beta}-latest` are removed.
In nightly 2022-02-02, aarch64-apple-darwin's rust-docs actually points
to x86_64-apple-darwin's artifact URL. Most tier 2 targets'
rust-docs also forward to -unknown-linux-gnu with the same arch.
Not sure why they do so, but URL forwarding is implemented now.
