flake: add attr minimal to each package

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
```

flake.nix

@@ -56,27 +56,34 @@
  # TODO: Flake outputs except `overlay[s]` are not stabilized yet.
 
  packages = let
- select = version: comps: if version == "latest" then null else comps.default or null;
+ select = version: comps:
+ if comps ? default then
+ comps.default // {
+ minimal = comps.minimal or (throw "missing profile 'minimal' for ${version}");
+ }
+ else
+ null;
  result =
  mapAttrs' (version: comps: {
- name = "rust_${replaceStrings ["."] ["_"] version}";
+ name = if version == "latest"
+ then "rust"
+ else "rust_${replaceStrings ["."] ["_"] version}";
  value = select version comps;
  }) pkgs.rust-bin.stable //
  mapAttrs' (version: comps: {
- name = "rust-nightly_${version}";
+ name = if version == "latest"
+ then "rust-nightly"
+ else "rust-nightly_${version}";
  value = select version comps;
  }) pkgs.rust-bin.nightly //
  mapAttrs' (version: comps: {
- name = "rust-beta_${version}";
+ name = if version == "latest"
+ then "rust-beta"
+ else "rust-beta_${version}";
  value = select version comps;
- }) pkgs.rust-bin.beta //
- rec {
- rust = pkgs.rust-bin.stable.latest.default;
- rust-beta = pkgs.rust-bin.beta.latest.default;
- rust-nightly = pkgs.rust-bin.nightly.latest.default;
- default = rust;
- };
- in filterAttrs (name: drv: drv != null) result;
+ }) pkgs.rust-bin.beta;
+ result' = filterAttrs (name: drv: drv != null) result;
+ in result' // { default = result'.rust; };
 
  checks = let
  inherit (pkgs) rust-bin rustChannelOf;