Update minimum tested Rust version to 1.64.

Some older toolchains for MacOS seem to be broken. Instead of trying to
pinpoint the exact version that works, we just go to the Rust version in
stable Alpine Linux.

Author: partim

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest, macOS-latest]
-        rust: [1.42.0, stable, beta, nightly]
+        rust: [1.64.0, stable, beta, nightly]
     steps:
     - name: Checkout repository
       uses: actions/checkout@v1