[lxd] long startup delay due to image refresh

[Saviq]

When the cached image is out of date, Multipass will try and update it on startup, causing a significant delay until instances are accessible.

$ snap remove --purge multipass
multipass removed

$ snap install multipass --stable
multipass 1.8.0 from Canonical✓ installed

$ sudo multipass set local.driver=lxd

$ multipass start
Launched: primary                                                               
Mounted '/home/michal' into 'primary:Home'                                      

$ multipass launch daily:devel;
Launched: absolute-shiner                                                       

$ multipass ls                 
Name                    State             IPv4             Image
primary                 Running           10.16.50.113     Ubuntu 20.04 LTS
absolute-shiner         Running           10.16.50.93      Ubuntu 22.04 LTS
                              
$ snap refresh multipass --channel candidate
multipass (candidate) 1.9.0-rc.442+g6301a89d from Canonical✓ refreshed

$ multipass ls                              
No instances found.

# after some seconds

$ multipass ls
Name                    State             IPv4             Image
primary                 Running           10.16.50.113     Ubuntu 20.04 LTS
absolute-shiner         Running           10.16.50.93      Ubuntu 20.04 LTS

A number of potential improvements:

• only invalidate the images (next instance launch will require a downloaded image)
• defer the image update
• run the image update in parallel (that would probably be the ultimate fix)

[Saviq]

In the exact above case this issue was amplified by #2495, which causes all images invalidate.