tests(ipv6): Do not run IPv6 tests if the host doesn't support IPv6 (#…

…1059)

Seems some of the cross environments simply don't do IPv6 at all.  In
that case there's no point in supporting IPv6.

.github/workflows/ci.yml

@@ -210,7 +210,7 @@ jobs:
       with:
         submodules: recursive
 
-    - name: Install nightly
+    - name: Install Rust 1.67
       # See /~https://github.com/cross-rs/cross/issues/1222
       uses: dtolnay/rust-toolchain@1.67