gh-actions: disable clang-3.9 build
An update to libstdc++ seems to have broken it.
@@ -265,9 +265,9 @@ jobs:
# - version: "3.8"
# distro: ubuntu-16.04
# arch_flags: -mavx2
- - version: "3.9"
- distro: ubuntu-18.04
- arch_flags: -mavx2
+ # - version: "3.9"
+ # distro: ubuntu-18.04
+ # arch_flags: -mavx2
# - version: "4.0"
# distro: ubuntu-18.04
# arch_flags: -mavx2
