Allow overriding DefaultIndexURI from an env variable

[corneliusweig]

We want to give operators who do not want to install from the default krew index the possibility to provide their own release branch of krew. For that, we should allow overriding the DefaultIndexURI from an env variable, e.g. KREW_DEFAULT_INDEX_URI.
Then operators can start using their own index (containing some krew plugin) from scratch. This will then also set up the default index and point to this custom index location.

• Implement override via env variable.
• Update the documentation for custom indices.

/cc @chriskim06

[ahmetb]

I'm supportive. This likely only benefits people who never want to touch krew’s own default.

[chriskim06]

Is this something we need before 0.4.0?

[ahmetb]

No, I think it can be done later.

[chriskim06]

Been busy this week, but I'll try to take a crack at this this weekend. I have a feeling integration testing might be interesting but I haven't taken a close look yet.

[chriskim06]

This completely fell off my radar, I'll try to get to this soon

/assign