Tool cache fixes

[ursetto]

This fixes a couple bugs in the tool cache:

• pyenv-archive.zip will not be redownloaded if it is already cached.
• pyenv_root will not be recreated (through reinstalling pyenv) if it is already cached. This avoids wiping out links to previously cached python installs, causing jobs to fail on persistent runners.

One of the integration tests is modified to use this action twice in the same job, which triggers the bug in the old code but is fixed by this PR. A persistent runner is not needed for this test.

The npm tests are left alone, as they do not currently install or test any python version after installing pyenv, and that's the only way to trigger a failure.

Resolves #320.

[gabrielfalcao]

Hey this is awesome!
Thanks for the PR.

I didn't think other human beings beside myself found much utility for this github action because most of the PRs I'd seen come from bots.
With that said, I'm wondering if you and @blakewatters would like to start a tiny community to maintain this and I'd move the project out of my personal account into a github org.

In the mean time I'll just merge both of your PRs and make a new release.

Thanks!