Codify linting toolchain in a toolchain file

Moves the nightly toolchain we use for linting out of a GitHub action
and into a dedicated `toolchains/lint/rust-toolchain.toml` file.

rustup doesn't currently support specifying a toolchain file, though
[it will soon](rust-lang/rustup#2686), so we
hack around it by moving the toolchain file back and forth as part of
the GitHub action for Rust.

.github/workflows/rust.yml

@@ -31,7 +31,6 @@ permissions:
   contents: read
 
 jobs:
-
   test:
     runs-on: ubuntu-latest-16-cores
     timeout-minutes: 45
@@ -103,11 +102,10 @@ jobs:
       run: >
         find linera-* -name '*.rs' -a -not -wholename '*/target/*' -print0
         | xargs -0 -L1 ./scripts/target/release/check_copyright_header
+    - name: Put lint toolchain file in place
+      run: |
+        ln -sf toolchains/lint/rust-toolchain.toml
     - uses: Twey/setup-rust-toolchain@v1
-      with:
-        toolchain: nightly-2023-10-22
-        target: wasm32-unknown-unknown
-        components: clippy rustfmt
     - name: Install cargo-machete
       run: |
         cargo install cargo-machete --locked
@@ -126,7 +124,7 @@ jobs:
         cargo install cargo-sort --locked
     - name: Check formatting
       run: |
-        cargo +nightly-2023-10-22 fmt -- --check
+        cargo fmt -- --check
     - name: Check for unused dependencies
       run: |
         cargo machete
@@ -161,3 +159,7 @@ jobs:
       run: |
         cd examples
         cargo sort -c -w
+    - name: Restore `rust-toolchain.toml` file
+      if: '!cancelled()'
+      run: |
+        ln -sf toolchains/build/rust-toolchain.toml

toolchains/build/rust-toolchain.toml

@@ -0,0 +1,5 @@
+[toolchain]
+channel = "1.73.0"
+components = [ "clippy", "rustfmt", "rust-src" ]
+targets = [ "wasm32-unknown-unknown" ]
+profile = "minimal"

toolchains/lint/rust-toolchain.toml

@@ -0,0 +1,4 @@
+[toolchain]
+channel = "nightly-2023-10-23"
+components = [ "clippy", "rustfmt" ]
+profile = "minimal"