Codify linting toolchain in a toolchain file

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

@@ -102,11 +102,11 @@ 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 rust-toolchain.lint.toml rust-toolchain.toml
     - uses: Twey/setup-rust-toolchain@v1
-      with:
-        toolchain: nightly-2023-10-22
-        target: wasm32-unknown-unknown
-        components: clippy rustfmt
+      working-directory: toolchains/lint
     - name: Install cargo-machete
       run: |
         cargo install cargo-machete --locked

rust-toolchain.toml

@@ -0,0 +1 @@
+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"