Thanks, this is a good starting point. The file imports the Target type from the google/syzkaller repo:
And there is indeed no CrossCFlags field defined.
The error message points to file
tools/syz-env/env.go
which looks like belonging to syzkaller but the repo does not seem to contain that path.
I’d reckon that there is something wrong with either of the two repos, maybe a mismatch of checked-out branches or commits.