Replace ATiltedTree/setup-rust with rust-toolchain

- https://github.com/ATiltedTree/setup-rust has not been committed to since May
  2022, I am uncertain about how widely used it is.
- It appears to have a bug with restoring its internal cache whenever there
  comes a new stable version (immediate guess would be the cache-key does not
  resolve `stable` to a specific version, which somehow breaks rustup, but I have not investigated)
- https://github.com/dtolnay/rust-toolchain is a more sensible take of https://github.com/actions-rs/toolchain,
  where the original repo appears to be unmaintained.
  It is implemented in one file of yaml/bash
  https://github.com/dtolnay/rust-toolchain/blob/master/action.yml, we could
  easily fork it if it becomes unmainted, unlike the other actions which uses
  unnecessary javascript

.github/workflows/rust_checks.yml

@@ -11,10 +11,7 @@ jobs:
 
     steps:
     - uses: actions/checkout@v3
-    - name: SetupRust
-      uses: ATiltedTree/setup-rust@v1
-      with:
-        rust-version: stable
+    - uses: dtolnay/rust-toolchain@stable
     - name: cargo fmt
       run: cargo fmt --check --all
 
@@ -23,10 +20,7 @@ jobs:
 
     steps:
     - uses: actions/checkout@v3
-    - name: SetupRust
-      uses: ATiltedTree/setup-rust@v1
-      with:
-        rust-version: stable
+    - uses: dtolnay/rust-toolchain@stable
     - name: Install deps
       run: |
         sudo apt install gettext libncurses5-dev libpcre2-dev python3-pip tmux