Its kind of hard to test. The easiest I have managed has been to see that I don't get the error with --emulate-fhs in my call, and the script that fails starts with:
#!/usr/bin/env txr To simplify things, I made a self contained txr script and wrapped that up in my test R package. Sure enough, after building the package I checked its store directory. Guix did not patch the path to the path to the env utility in that case.