ci: bump pinned version to Rust 1.70

Dependencies are using features that require a newer Rust. Since this
crate doesn't have an MSRV policy, I'm just going to bump the pinned
version in CI to `{latest} - 2` and move on with my life.

Author: BurntSushi

.github/workflows/ci.yml

@@ -23,7 +23,7 @@ jobs:
         include:
         - build: pinned
           os: ubuntu-latest
-          rust: 1.60.0
+          rust: 1.70.0
         - build: stable
           os: ubuntu-latest
           rust: stable