[macos] Setting MULTIPASS_STORAGE via the docs does not work on newer macOS

[townsend2010]

A user reported on IRC that their instances were no longer seen after a macOS update. They had set MULTIPASS_STORAGE as instructed in the docs. After some investigation, it has been determined that MULTIPASS_STORAGE is not being set.

This has been reproduced on 13.4.1 and 13.4. It seems some new changes to the System Integrity Protection (SIP) was introduced in 13.4, but this is merely anecdotal at this time. More investigation on when this behavior began is needed and also to figure out a way to mitigate this.

[townsend2010]

I missed this, but it has been reported at #2417 (comment) and #2417 (comment).

Long term, the answer is to add a setting that changes the storage directory and migrates existing data to that directory. Shorter term, documenting the symlink workaround is best.

[giuliazanchi]

The workaround mentioned in #2417 (comment) and #2417 (comment) is documented in https://multipass.run/docs/configure-multipass-storage.

@ricab see if we can close this based on Chris' comment above.

[ricab]

Yes, I think so. Thanks!