build_tools/style.fish: check that {rustfmt,Cargo}.toml edition specs are in sync

Commit 1fe6b28877 (rustfmt.toml: specify edition to allow 2024 syntax,
2025-10-19) mentions that "cargo fmt" has different behavior than
"rustfmt" before that commit.  Probably because when .rustfmt.toml
exists, rustfmt implicitly uses a different edition (2018?)  that
doesn't support c"" yet.  That commit bumped the edition to 2024,
which caused yet another deviation from "cargo fmt":

    Error writing files: failed to resolve mod `tests`: cannot parse /home/johannes/git/fish-shell/src/wutil/tests.rs
    error: expected identifier, found reserved keyword `gen`
      --> /home/johannes/git/fish-shell/src/tests/topic_monitor.rs:48:9
       |
    48 |     for gen in &mut gens_list {
       |         ^^^ expected identifier, found reserved keyword

This has since been fixed by
00784248db (Update to rust 2024 edition, 2025-10-22).

Let's add a test so that such changes won't randomly break "rustfmt"
again.

Fix that by using 2021 edition, like we do in Cargo.toml.

In future, rustfmt should probably default to a current edition (or
maybe read the edition from Cargo.toml?)  Not yet sure which one is the
upstream issue, maybe https://github.com/rust-lang/rustfmt/issues/5650

.github/workflows/lint.yml

@@ -19,6 +19,9 @@ jobs:
       run: cargo build
     - name: check format
       run: PATH="target/debug:$PATH" build_tools/style.fish --all --check
+    - name: check rustfmt
+      run: find build.rs crates src -type f -name '*.rs' | xargs rustfmt --check
+
 
   clippy:
     runs-on: ubuntu-latest