system_command_spec: Make part of bash error regex optional

[rwhogg]

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brew tests fails on my system without this. Bash has two very similar error messages that can come up when running the command specified by this test. Here's the relevant code from subst.c in a recent version of Bash:

  else if (check_null == 0)
    report_error (_("%s: parameter not set"), name);
  else
    report_error (_("%s: parameter null or not set"), name);

[MikeMcQuaid]

Good catch, thanks @rwhogg!