Bump CI toolchain from "1.78.0" to "1.86.0" (as required by tryexpand@0.12.0)

Author: regexident

.github/workflows/rust.yml

@@ -41,7 +41,7 @@ jobs:
       - uses: dtolnay/rust-toolchain@v1
         with:
           # update `cargo-expand` accordingly, when bumping MSRV:
-          toolchain: 1.78.0
+          toolchain: 1.86.0
           components: rust-src
       - uses: cargo-bins/cargo-binstall@v1.15.4
       - run: cargo binstall cargo-expand@1.0.95 --no-confirm