Switch msrv in CI back to 1.64

Author: akern40

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
           - stable
           - beta
           - nightly
-          - 1.65.0  # MSRV
+          - 1.64.0  # MSRV
     steps:
       - uses: actions/checkout@v4
       - uses: dtolnay/rust-toolchain@master