syncthing fails to build on macOS 10.13, odd UTF-8 error

[Greg Troxel <gdt%lexort.com@localhost>]

Trying to build net/syncthing on macOS 10.13, I get

  ===> Checking for vulnerabilities in syncthing-1.22.0nb2
  ===> Overriding tools for syncthing-1.22.0nb2
  ===> Extracting for syncthing-1.22.0nb2
  error:  cannot create github.com/maruel/panicparse@v1.6.2/cmd/panic/internal/utf8/+tf8.go
          Illegal byte sequence
  *** Error code 50

and it seems the zip for panicparse has #xc3 #xb9 in it.   On NetBSD I
get a file with a literal ? and on mac I get the above error.

Is the fix to set LC_CTYPE to C?  Is this a bug someplace else?