Honor the "debug" kernel cmdline option in Tails Installer

As we’ve done with the Tails greeter (see #12373 (closed)), tails-installer should output its debug logs in the journal if the debug option is set in /proc/cmdline.

Related issues

Related to #12399 (closed)
Related to #12373 (closed)
Related to #12395

Original created by @bertagaz on 12396 (Redmine)

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