Override with nightly toolchain

Author: josephlr

.github/workflows/tests.yml

@@ -227,6 +227,7 @@ jobs:
           profile: minimal
           toolchain: nightly
           components: rust-src
+          override: true
       - uses: Swatinem/rust-cache@v1
       - name: Build and Link tests (build-std)
         # This target is Tier 3, so we have to build libstd ourselves.