rustfmt: use nightly in CI

This PR opts for using the `nightly` channel in CI for `rustfmt`.

Author: Joe C

.github/workflows/pull-request.yml

@@ -28,11 +28,11 @@ jobs:
       - name: Set env vars
         run: |
           source ci/rust-version.sh
-          echo "RUST_STABLE=$rust_stable" >> $GITHUB_ENV
+          echo "RUST_NIGHTLY=$rust_nightly" >> $GITHUB_ENV
 
       - uses: actions-rs/toolchain@v1
         with:
-          toolchain: ${{ env.RUST_STABLE }}
+          toolchain: ${{ env.RUST_NIGHTLY }}
           override: true
           profile: minimal
           components: rustfmt

cargo-nightly.sh

@@ -0,0 +1,8 @@
+#!/usr/bin/env bash
+
+here=$(dirname "$0")
+source "${here}"/ci/rust-version.sh nightly
+# shellcheck disable=SC2054 # rust_nightly is sourced from rust-version.sh
+toolchain="$rust_nightly"
+set -x
+exec cargo "+${toolchain}" "${@}"

rustfmt.toml

@@ -0,0 +1 @@
+edition = "2021"