env: reraise signal from child process

[tertsdiepraam]

Closes #3188

This PR reraises any signals from the child process instead of printing them. This is more in line with GNU behaviour. The message that we previously printed was actually not GNU behaviour, but part of the shell.

I'm not sure how to test this in the CI. I need to spawn a child process that raises a signal to respond to. In normal testing I used a custom C program, but I'm not sure how to get that compiled in the CI:

#include <signal.h>
#include <stdlib.h>

int main(int argc, char** argv) {
    raise(atoi(argv[1]));
}