update flake.nix (#6200)

Before I was getting
```
error: attribute 'targetPlatforms' missing
at /nix/store/dyzl40h25l04565n90psbhzgnc5vp2xr-source/pkgs/build-support/rust/build-rust-package/default.nix:162:7:
  161|       meta.platforms or lib.platforms.all
  162|       rustc.targetPlatforms;
     |       ^
  163|   };
```
This was probably an upstream issues as discussed in here
https://discourse.nixos.org/t/error-attribute-targetplatforms-missing-after-updating-inputs/54494

After this update it is fixed.