multipass set homecache=yes

[abitrolly]

What are you trying to do?
Free up disk space on root / volume.

What's your proposed solution?
Add configuration option to move /var/snap/multipass/common/cache to $HOME/.cache/multipass.

Additional context
#2417 discusses configuring directory for images, but network cache in data8 dir can also contain files that could be cleaned.

$ tree /var/snap/multipass/common/cache
/var/snap/multipass/common/cache
└── multipassd
    ├── multipass-blueprints.zip
    ├── network-cache
    │   ├── data8
    │   │   ├── 0
    │   │   ├── 1
    │   │   ├── 2
    │   │   │   ├── 2spnnkj2.d
    │   │   │   └── 347wq17b.d
    │   │   ├── 3
    │   │   │   ├── 22w41de3.d
    │   │   │   ├── 2jtnwbdc.d
    │   │   │   ├── 2nhgehjc.d
    │   │   │   └── 3ht1vk5s.d
    │   │   ├── 4
    │   │   ├── 5
    │   │   │   └── 1gqs6xlu.d
    │   │   ├── 6
    │   │   │   └── 1n5xhdff.d
    │   │   ├── 7
    │   │   ├── 8
    │   │   │   ├── 15c3dxbh.d
    │   │   │   └── 1rpg6n38.d
    │   │   ├── 9
    │   │   │   └── 1j6cmzg9.d
    │   │   ├── a
    │   │   │   ├── 2jsru05j.d
    │   │   │   ├── 8na2pa9z.d
    │   │   │   └── b84d7dpj.d
    │   │   ├── b
    │   │   ├── c
    │   │   ├── d
    │   │   │   └── r97fxz5m.d
    │   │   ├── e
    │   │   └── f
    │   └── prepared
    └── vault
        ├── images
        │   └── snapcraft-core18-20201111
        │       └── bionic-server-cloudimg-amd64-disk.img
        └── multipassd-image-records.json

With cache it should be an easy decision to drop directory without worrying about user instances.