install: Check if running in container earlier

[ckyrouac]

Previously, the check to verify install to-filesystem is being run within a container happened fairly late in prepare_install. This moves the check up before some of the other container specific checks. Now, the error should occur when trying a basic install to-filesystem invocation, e.g. bootc install to-filesystem /var/mnt.

This also adds a test to verify the error occurs with minimal arguments and adds host_is_container to the global state.

[cgwalters]

What we also should do here I think is add a test case...mmm in the nushell bits should be easy that verifies we do currently get an error.

[ckyrouac]

reworked this to focus on more clearly validating if install to-filesystem is running in a container.