ci: standardize go mod vendor

[jackfrancis]

Reason for Change:

This PR standardizes our usage of the vendor directory so that both the aks-engine code itself and the E2E go module store local dependencies in vendor/.

Also ships a GitHub Action to regularly maintain go mod vendor, and it replaces the (now no longer needed) hack/tools GitHub Action.

Issue Fixed:

Credit Where Due:

Does this change contain code from or inspired by another project?

• No
• Yes

If "Yes," did you notify that project's maintainers and provide attribution?

• No
• Yes

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

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This pull-request has been approved by: jackfrancis

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