chore: pin once-cell on msrv ci job (#175)

Author: seanmonstar

.github/workflows/CI.yml

@@ -71,6 +71,7 @@ jobs:
           cargo update --package tokio --precise 1.38.1
           cargo update --package tokio-util --precise 0.7.11
           cargo update --package hashbrown --precise 0.15.0
+          cargo update --package once_cell --precise 1.20.3
       - name: Install Rust (${{ matrix.rust }})
         uses: dtolnay/rust-toolchain@master
         with: