Pin rayon-core in CI for MSRV compatibility.

Author: sunfishcode

.github/workflows/main.yml

@@ -60,6 +60,7 @@ jobs:
         cargo update --package=parking_lot --precise=0.12.3
         cargo update --package=parking_lot_core --precise=0.9.10
         cargo update --package=lock_api --precise=0.4.12
+        cargo update --package=rayon-core --precise=1.12.1
 
     - run: >
         rustup target add
@@ -543,6 +544,7 @@ jobs:
         cargo update --package=parking_lot --precise=0.12.3
         cargo update --package=parking_lot_core --precise=0.9.10
         cargo update --package=lock_api --precise=0.4.12
+        cargo update --package=rayon-core --precise=1.12.1
 
     - run: |
         cargo test --verbose --features=all-apis --release --workspace -- --nocapture