Raise minimum rustc to 1.64

Required by clap 4.1 / clap_lex 0.3.1:

    error: package `clap_lex v0.3.1` cannot be built because it requires rustc 1.64.0 or newer

.github/workflows/ci.yml

@@ -37,7 +37,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        rust: [nightly, beta, stable, 1.60.0]
+        rust: [nightly, beta, stable, 1.64.0]
     timeout-minutes: 45
     steps:
       - uses: actions/checkout@v3