Use Nightly + Cranelift for dev, only fail on warnings in CI (#4388)

* Switch to nightly + cranelift

* Fail on warnings only in CI

* Fix check errors

* Don't use mold on Linux to fix CI

* Pin nightly toolchain and add default rustup components

* Fix another CI thing

* PR comment

.github/workflows/turbo-ci.yml

@@ -18,6 +18,11 @@ jobs:
       FORCE_COLOR: 3
       # Make cargo nextest successfully ignore projects without tests
       NEXTEST_NO_TESTS: pass
+      # Fail on warnings in CI
+      # (but don't do this in the root `Cargo.toml`,
+      # since we don't want warnings to become errors
+      # while developing)
+      RUSTFLAGS: -Dwarnings
 
     steps:
       - name: 📥 Check out code