Snaps failing on an Ubuntu 22.04 ZFS system because of missing systemd unit files

Snaps started being listed as `broken` after rebooting.

After a lot of trouble, I discovered that the `squashfs` file that contains the snap was failing to be mounted, because the ZFS partition that it was on hadn't finished mounting.

My hypothesis is that snapd `2.58` (installed as a snap) when *combined* with snap `2.57` (as installed by apt in Ubuntu 22.04) fails on ZFS systems because the unit file required that ensures `zfs.mount.service` is read (`snapd.mounts-pre.target`) is never put in place by the *snap* for snapd `2.58`.

```
tak# cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=22.04
DISTRIB_CODENAME=jammy
DISTRIB_DESCRIPTION="Ubuntu 22.04.1 LTS"
```

```
tak# snap --version
snap 2.58
snapd 2.58
series 16
ubuntu 22.04
kernel 5.15.0-58-generic
```

```
tak# snap list | grep snapd
snapd 2.58 17950 latest/stable canonical** snapd
```

```
tak# apt-cache policy snapd
snapd:
  Installed: 2.57.5+22.04ubuntu0.1
  Candidate: 2.57.5+22.04ubuntu0.1
  Version table:
 *** 2.57.5+22.04ubuntu0.1 500
        500 http://nz.archive.ubuntu.com/ubuntu jammy-updates/main amd64 Packages
        500 http://security.ubuntu.com/ubuntu jammy-security/main amd64 Packages
        100 /var/lib/dpkg/status
     2.55.3+22.04 500
        500 http://nz.archive.ubuntu.com/ubuntu jammy/main amd64 Packages
```

Ubuntu 22.04 installs `2.57`, but then Snap updates itself with the snap for `2.58`.

In this situation, the `.mount` file generated includes the clause `After=snapd.mounts-pre.target` but the actual unit file for `snapd.mounts-pre.target` is *not* installed, and isn't present in `/etc/systemd/system` (or wherever it's supposed to go, I'm not an expert at systemd)

Unit files with the right configuration *do* exist in `/snap/snapd/` but they're not installed into a directory where systemd will process them:

```
tak# find /snap/snapd/current/ -name "*.target"
/snap/snapd/current/lib/systemd/system/snapd.mounts-pre.target
/snap/snapd/current/lib/systemd/system/snapd.mounts.target
```

So the fix for my system was to manually copy these files to `/etc/systemd/system/`

I think, if the snap for `snapd` is going to be installed to "upgrade" it, it'll need to put these systemd files in the right place or it'll continue to fail for Ubuntu systems running on ZFS.