Issue warning message when using wrong LIMA_HOME

[afbjorklund]

Improves the error message, when using a top-level directory by mistake.

$ mkdir -p /tmp/foo/bar
$ LIMA_HOME=/tmp/foo _output/bin/limactl ls
WARN[0000] The directory "/tmp/foo" does not look like a valid Lima directory: stat /tmp/foo/bar/lima.yaml: no such file or directory 
FATA[0000] unable to load instance bar: open /tmp/foo/bar/lima.yaml: no such file or directory 

We want to avoid the case where people "forget" to include the lima subdirectory.

• #2325

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Currently the semantics are hidden under "internals": https://lima-vm.io/docs/dev/internals/

And it can be assumed that LIMA_HOME will work similar to HOME, i.e. add the subdir for you

Once we have documentation on how to relocate the instances to a local disk, we can recommend a subdir.

Something like LIMA_HOME=/local/$USER/lima or somesuch, for where ~/.lima is on a network filesystem