CI: disable tests against Julia nightly

It is not helpful to always have these failing for reasons
beyond our control. For background information, see here:
<JuliaLang/julia#55006>

.github/workflows/CI.yml

@@ -30,7 +30,7 @@ jobs:
  - '1.9'
  - '1.10'
  - '~1.11.0-0'
- - 'nightly'
+ #- 'nightly' # disabled due to https://github.com/JuliaLang/julia/issues/55006
  os:
  - ubuntu-latest
  #include: