Disable clippy beta on CI

[0] triggers a false-positive that prevents progress. Since I have no
good idea what to do about that, my bad idea is to just disable this
part of CI:

[0]: #561
[1]: rust-lang/rust#79498

Signed-off-by: Uli Schlachter <psychon@znc.in>

.github/workflows/CI.yml

@@ -31,7 +31,8 @@ jobs:
        matrix:
          include:
            - rust: stable
-           - rust: beta
+           #fixme: beta was disabled due to the false positive from https://github.com/rust-lang/rust/issues/79498
+           #- rust: beta
              #fixme: remove this hack
              clippy_args: -A clippy::manual_strip
      steps: