{rust-bin,example}: remove WASI target workaround and fix example

The un-versioned `wasm32-wasi` target is removed in favor of
`wasm32-wasip?` since Rust 1.84. Examples are updated.

The nixpkgs patch for WASI target name remapping is merged in 24.11.
We can also safely remove the workaround.

flake.nix

@@ -29,7 +29,6 @@
       pkgs:
       lib.fix (rust-bin: import ./lib/rust-bin.nix {
         inherit lib pkgs;
-        inherit (pkgs.rust) toRustTarget;
         inherit (rust-bin) nightly;
         manifests = mkManifests distRoot;
       });