Mounts: prohibit relative paths in YAML (while allowing them in `limactl create --mount` CLI)

[AkihiroSuda]

The current YAML surprisingly allows relative paths (maybe a regression):

mounts:
- location: .

The actual path changes depending on the cwd of the limactl start command.

In Lima v2.0, these relative paths should be prohibited to make the command deterministic.

limactl create --mount DIR should continue to support relative paths.
A relative path should be resolved into an absolute path at the time of limactl create.

[AkihiroSuda]

The current code seems already prohibiting relative paths, but the validation seems not working

lima/pkg/limayaml/validate.go

Lines 108 to 111 in 11a81a8

	if !filepath.IsAbs(f.Location) && !strings.HasPrefix(f.Location, "~") {
	errs = errors.Join(errs, fmt.Errorf("field `mounts[%d].location` must be an absolute path, got %q",
	i, f.Location))
	}

This is not a v2 regression. The issue is reproducible with v1.2.1 too.