Add env var and logic to skip git syncing

[ryanmerolle]

When testing, it would be useful to skip infrahub git syncing.

[petercrocker]

I'm assuming for your testing you just want schema and the bootstrap data loaded?

One of the most common uses of this demo is showing generators automatically running when an object gets created, and that's based on events that are pulled in from the linked git repo. But I could see where loading a subset would be useful for testing, and done as an environmental variable or different invoke command could be useful.