update rustc in CI to 1.64 (#495)

Tests for the kafkaesque example started failing because it depends on
`clap` which now requires rustc `1.64`. We could change the deps of the
example but updating rustc seems like a good anything anyway.

Signed-off-by: Petros Angelatos <petrosagg@gmail.com>

Signed-off-by: Petros Angelatos <petrosagg@gmail.com>

Author: petrosagg

.github/workflows/test.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
     - uses: actions/checkout@v3
-    - run: rustup update 1.60 --no-self-update && rustup default 1.60
+    - run: rustup update 1.64 --no-self-update && rustup default 1.64
     - run: cargo build
     - name: test mdBook
       # rustdoc doesn't build dependencies, so it needs to run after `cargo build`,