systemd/system/sshkeys: Make execution more robust

[pothos]

The service failed when the unit to start was masked. Since it involves
networking, the service can fail and if it does we should restart. It
also seems that the service relied on
update-ssh-keys-after-ignition.service to run update-ssh-keys once to
create the subfolder and could possibly race with it.

Prevent execution when it has no chance to succeed because the unit to
start is masked but in all other cases try to restart on failure. Also
order behind update-ssh-keys-after-ignition.service.

How to use

Backport

Testing done

with

variant: flatcar
version: 1.0.0
systemd:
  units:
    - name: sshkeys.service
      dropins:
        - name: test.conf
          contents: |
            [Unit]
            After=update-ssh-keys-after-ignition.service
            ConditionPathIsSymbolicLink=!/etc/systemd/system/coreos-metadata-sshkeys@core.service
            [Service]
            Restart=on-failure

and verifying that the service does not run when the unit to start is masked.
@tormath1 also checked that the spurious failure experienced when update-ssh-keys-after-ignition.service is slower than sshkeys.service didn't occur again.

• Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.