bootupctl should pass RUST_BACKTRACE=... to the systemd unit when set for the command line call

[travier]

In fedora-silverblue/issue-tracker#602, the user tried to get more logs but it looks like the env variable did not get passed to the systemd run invocation.

Temporary workaround should be to add a drop-in config with the env var:

$ cat /etc/systemd/system/bootupd.service.d/debug.conf
[Service]
Environment=RUST_BACKTRACE=full

[HuijingHei]

Yes, agree that we should support this, it is helpful to debug.