optimize: don't parse Dockerfile twice, reusing stages

[dani29]

Description

Planning to refactor a bit to work on issue #533, this one is just doing preparation refactoring that is out of scope for that issue.

• We call CalculateDependencies(), which parsing the Dockerfile again into kanikoStages, even though we already have them.
• modified command_utils.UpdateConfigEnv() so that it will only update the config with the state of the existing environment variables, but will not modify the cmd.Env object itself. It prevents escaping the same ENV command twice (during calculateDependencies, and ExecuteCommand).

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Reviewer Notes

• The code flow looks good.
• Unit tests and or integration tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

Examples of user facing changes:
- Optimization: do not parse Dockerfile again while calculating dependencies

[tejal29]

Looks good to me!