systemd v261 lists the new systemd-boot-fallback binary in `bootctl
status`, so our regex picked up the older fallback version and ran
`bootctl update` on an already current ESP, which then exits non-zero.
Switch from parsing the `bootctl --status` output to having bootctl
compare versions itself by calling io.systemd.BootControl.Install over
Varlink. bootctl is spawned as a stdio Varlink server so that we always
use the bootctl from the target closure. An already current binary comes
back as an io.systemd.System error carrying ESTALE, which we treat as
success.
r-vdp
bb258c2dbe