Update reboot-required file with information about the staged image

[bshephar]

This change updates the /run/reboot-required file to include information
about the staged bootc image. This will provide users with information
about what bootc image will be effective after the system has been
rebooted.

Fixes #1574

[cgwalters]

Thanks for picking this up!

[bshephar]

I didn't realize that we were already creating this file, so this change is much more simple than I had initially planned. I think it should always be updated in that stage function, so we don't need a helper and to handle it for each operation.

[bshephar]

Result looks like:

❯ sudo target/release/bootc upgrade
layers already present: 67; layers needed: 1 (695.2 MB)
Fetched layers: 662.96 MiB in 34 seconds (19.51 MiB/s)
  Deploying: done (6 seconds)
Queued for next boot: ghcr.io/bshephar/fedora-asahi-bootc:42
  Version: 42.20250904.0
  Digest: sha256:8ea48cc32573500a367526e687cd61510859e097df82402bf352d98da755c6c9
Total new layers: 68    Size: 3.2 GB
Removed layers:   1     Size: 688.5 MB
Added layers:     1     Size: 695.2 MB

❯ cat /run/reboot-required
Reboot required for bootc image: ghcr.io/bshephar/fedora-asahi-bootc:42

I guess if this was a bootc switch instead of a bootc upgrade it would be more useful, because the image would be different to the one I'm currently running. But because I have used a tag, this is always just going to say the same thing. Do we feel like that is still valuable? Or, do we realistically want the context of which operation has staged the image?

Maybe there's something more informative than just &spec.image.image like I'm using here?

Edit: Maybe image.manifest_digest.as_ref() would be better here actually:
https://github.com/bootc-dev/bootc/blob/main/crates/lib/src/deploy.rs#L668

Using the digest looks like:

❯ cat /run/reboot-required
Reboot required for bootc image digest: sha256:c0aaf49e1df7ec2f4baa4ed8077b684cad36820381f1a7511247558fb35e2bea%

[cgwalters]

Thanks!