Add env vars to kaniko specs

[cyrildiagne]

Rebase of #3115
Relates to #731

Fixes #731.

Description

This will give users the option to provide environment variables to the kaniko pod.
This should fix #731 and enable ECR support on regions other than the default one.

User facing changes

n/a

Next PRs.

n/a

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes unit tests
• Mentions any output changes.
• Adds documentation as needed: user docs, YAML reference, CLI reference.
• Adds integration tests if needed.

Reviewer Notes

• The code flow looks good.
• Unit test added.
• User facing changes look good.

Release Notes

[codecov]

Codecov Report

Merging #3389 into master will increase coverage by 0.01%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
pkg/skaffold/schema/latest/config.go	100% <ø> (ø)	⬆️
pkg/skaffold/build/cluster/sources/sources.go	91.89% <100%> (+0.3%)	⬆️