Pin once_cell to 1.14 to fix MSRV

Author: TheBlueMatt

.github/workflows/build.yml

@@ -36,6 +36,7 @@ jobs:
           cargo update -p tokio-postgres --precise "0.7.5" --verbose
           cargo update -p postgres-types --precise "0.2.3" --verbose
           cargo update -p tokio --precise "1.14.1" --verbose
+          cargo update -p once_cell --precise "1.14.0" --verbose
       - name: Build on Rust ${{ matrix.toolchain }}
         run: |
           cargo build --verbose --color always