test 'broken image errors' fails on Ubuntu 19.04

The broken image test does not work on Ubuntu 19. On centos7 the test completes normally.

✗ broken image errors (in test file run/ch-run_misc.bats, line 576) '[[ $output = *"can't execve(2): true: No such file or directory"* ]]' failed ch-run[3864]: can't execve(2): true: Not a directory (charliecloud.c:437 20)

The output for execve(2) in Ubuntu 18 appears slightly different than the output expected.

Tested using the squashfs branch of charliecloud on both machines.