git-client-plugin tries to execute /bin/echo on unix systems.
On NixOS there is nothing inside the /bin folder except for /bin/sh. So the git-client-plugin fails everytime to execute this command.

NixOS has a very different approach how to handle dependencies (even coreutils which includes echo). However tools like echo are available in NixOS but in a different location which is added to the PATH environment variable.

Is there any specific reason why git-client-plugin uses a hard-coded path to the echo binary? For me the problem would be solved when git-client-plugin would rely on echo to be available by including it in PATH.
If that is not possible I would suggest to change implementation of git-client-plugin to look if /bin/echo exists if not it should fallback to echo.