feat: Update existing SDK environment variables

[masinger]

This enables processing of already admitted and modified containers. This might occur since the reinvocationPolicy has been set to IfNeeded #749.

Reason for Change: Not updating existing environment variables might result in a situation where the wrong credentials are projected into the container, whenever another admission plugin modifies the serviceAccountName.

Requirements

• [X] squashed commits
• [ ] included documentation
• [X] added unit tests and e2e tests (if applicable).

Issue Fixed: Fixes #1475

Please answer the following questions with yes/no:

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

• [X] no

If so, did you notify the maintainers and provide attribution? not relevant

[masinger]

@microsoft-github-policy-service agree company="lise GmbH"

[enj]

I am skeptical that is makes sense to support admission plugins that mutate serviceAccountName 🤔 @aramase WDYT?