Ensure support_conda_env uses `isystem`

[robertmaynard]

Description

Conda injects the path to the conda include directory as a system include when building via CXXFLAGS / CFLAGS / etc. We need to match this behavior so that we have consistent search order, since system includes are used last compared to user includes.

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.
• [x] The cmake-format.json is up to date with these changes.

[robertmaynard]

/merge