C++ headers not found on GitHub Actions Ubuntu 20210517.1
Last main commit which was working used

  Environment: ubuntu-20.04
  Version: 20210510.0
  Included Software: actions/virtual-environments@ubuntu20/20210510.0/images/linux/Ubuntu2004-README.md
  Image Release: actions/virtual-environments@ubuntu20%2F20210510.0 (release)

First broken commit was

  Environment: ubuntu-20.04
  Version: 20210517.1
  Included Software: actions/virtual-environments@ubuntu20/20210517.1/images/linux/Ubuntu2004-README.md
  Image Release: actions/virtual-environments@ubuntu20%2F20210517.1 (release)

https://github.com/actions/virtual-environments/blob/ubuntu20/20210510.0/images/linux/Ubuntu2004-README.md vs https://github.com/actions/virtual-environments/blob/ubuntu20/20210517.1/images/linux/Ubuntu2004-README.md has clang version changes, which are likely the cause.