The PATH of the build machine user is:
The build host is not a pure guix installation but also has guix on top of Debian Buster. It seems like the file transfer command is using an ssh call which doesn't setup the proper guix environment (as, e.g., the /usr/local/bin/guix script does). For example, PATH will have /usr/local/bin before the guix environment such that the wrong guile is selected.
Not sure how the proper setup should look like on the build host side...