Don't update the user's shell config files if VOLTA_HOME and PATH already contains what we need.

[dsully]

This will prevent changes when these settings are defined in a different, but compatible way to what volta does.

[charlespierce]

Thanks @dsully! Looks mostly good, just a couple of small tweaks to make sure things work as expected and are easier to debug later on.

Also, would you mind creating a follow-on issue to add a --force or similar flag? It's not necessary for this PR, but would be good to track the need for something like that in case we want to have the ability to force it through even given the environment appears to be properly set up.

[charlespierce]

Looks like the changes need to have cargo fmt run on them. Also the Windows builds are failing for some unrelated reason, it appears, I'll investigate that separately.

[dsully]

Done & done!

[charlespierce]

One more small change (that may need cargo fmt run again afterwards).

[charlespierce]

LGTM, thanks! The CI failures are unrelated and fixed in #993, once that's merged I'll merge this and then confirm that everything passes post-merge.