build-driver: hide build-driver traceback when grml-live fails

[zeha]

Currently a grml-live failure driven by build-driver has a very noisy traceback, looking like this:

  [!] Error: critical error while executing fai [exit code 2]. Exiting.
  [*] Cleaning up
E: Caught fatal exception
I: moving build results from /tmp/tmpkrp8sw9w to /builds/grml/build-daily/results
Traceback (most recent call last):
  File "/builds/grml/build-daily/grml-live/build-driver/build.py", line 526, in <module>
    sys.exit(main(sys.argv.pop(0), sys.argv))
             ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/builds/grml/build-daily/grml-live/build-driver/build.py", line 492, in main
    build(
    ~~~~~^
        build_dir,
        ^^^^^^^^^^
    ...<4 lines>...
        old_iso_path,
        ^^^^^^^^^^^^^
    )
    ^
  File "/builds/grml/build-daily/grml-live/build-driver/build.py", line 244, in build
    run_grml_live(
    ~~~~~~~~~~~~~^
        grml_live_path,
        ^^^^^^^^^^^^^^^
    ...<9 lines>...
        job_properties.job_timestamp,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/builds/grml/build-daily/grml-live/build-driver/build.py", line 138, in run_grml_live
    run_x(grml_live_cmd, env=env)
    ~~~~~^^^^^^^^^^^^^^^^^^^^^^^^
  File "/builds/grml/build-daily/grml-live/build-driver/build.py", line 54, in run_x
    return subprocess.run(args, check=check, **kwargs)
           ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.13/subprocess.py", line 577, in run
    raise CalledProcessError(retcode, process.args,
                             output=stdout, stderr=stderr)
subprocess.CalledProcessError: Command '['/builds/grml/build-daily/grml-live/grml-live', '-F', '-A', '-a', 'amd64', '-c', 'GRML_FULL,SOURCES', '-s', 'testing', '-v', 'd20260614b744', '-r', 'daily20260614build744testing', '-g', 'grml-full-amd64', '-i', 'grml-full-daily20260614build744testing-amd64.iso', '-o', '/tmp/tmpkrp8sw9w']' returned non-zero exit status 1.

Afterwards it should look like this:

  [!] Error: critical error while executing fai [exit code 2]. Exiting.
  [*] Cleaning up
E: Caught fatal exception
I: moving build results from /tmp/tmpkrp8sw9w to /builds/grml/build-daily/results
E: grml-live failed with exit code 1, command was: "/builds/grml/build-daily/grml-live/grml-live" "-F" "-A" "-a" "amd64" "-c" "GRML_FULL,SOURCES" "-s" "testing" "-v" "d20260614b744" "-r" "daily20260614build744testing" "-g" "grml-small-amd64" "-i" "grml-full-daily20260614build744testing-amd64.iso" "-o" "/tmp/tmpkrp8sw9w"

[zeha]

Builds are failing because of the known systemd-tmpfiles issue in forky (#533).