ci(template): Use nightly toolchain for rustfmt job

Author: Techassi

template/.github/workflows/build.yml.j2

@@ -27,6 +27,7 @@ env:
   CARGO_INCREMENTAL: '0'
   CARGO_PROFILE_DEV_DEBUG: '0'
   RUST_TOOLCHAIN_VERSION: "{[ rust_version }]"
+  RUST_NIGHTLY_TOOLCHAIN_VERSION: "{[ rust_nightly_version }]"
   PYTHON_VERSION: "{[ python_version }]"
   RUSTFLAGS: "-D warnings"
   RUSTDOCFLAGS: "-D warnings"
@@ -136,7 +137,7 @@ jobs:
           submodules: recursive
       - uses: dtolnay/rust-toolchain@c5a29ddb4d9d194e7c84ec8c3fba61b1c31fee8c
         with:
-          toolchain: ${{ env.RUST_TOOLCHAIN_VERSION }}
+          toolchain: ${{ env.RUST_NIGHTLY_TOOLCHAIN_VERSION }}
           components: rustfmt
       - run: cargo fmt --all -- --check