Use `INSTA_UPDATE=force` for forcing snapshot updates

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In an effort to simplify the configs, this merges the INSTA_UPDATE and INSTA_FORCE_UPDATE configs. Conceptually, INSTA_FORCE_UPDATE overwrites INSTA_UPDATE; they naturally fit into the same config setting.

I realized after starting this that we want to be careful about supporting new & old versions of cargo-insta. So this takes a conservative approach, only changing insta at first, but with the future updates to cargo-insta commented in the code. I realize that adds a bit of complication; though on balance I think simplifying the configs would be helpful and this makes a step towards that.

It stacks on https://github.com/mitsuhiko/insta/pull/479, which should merge first.

I'd be open to writing some tests for this if that'd be helpful.