Delay when creating an app using environment templates

[itowlson]

When you run spin new -E, it always fetches latest templates, which incurs a delay.

We would like it to not incur a delay.

The challenge with this is: the current behaviour ensures that the app you create is lined up with the environment definition. If we use old (cached) templates, it may not line up with the environment definition. So how do we ensure that templates are current, and yet not incur a delay when they are?

[itowlson]

We could alleviate this for envs that specify a Git commit hash for their templates, by recording the installed commit and comparing against the env file. But in practice envs are more likely to specify a tag or branch, and determining if a tag has moved is still a network operation - faster than a clone but still a perceptible delay. We could assume that tags are immutable: I dunno how dangerous that assumption would be. We don't currently distinguish in the env TOML between commit revs (immutable), branches (mutable), and tags (technically mutable but maybe immutableable by convention). So we can't tell if the tag really is a tag, or a sneaky branch masquerading.

The mitigation we use for unversioned envs is to check if they're up to date within X hours, and not refresh them if so; I guess we could use the same strategy here but man I didn't like it there and I wouldn't love having to use it more widely.