remove build args from composite key and replace all build args

[tejal29]

Fixes #1008

Description

In this change, we remove all the build-args being added to composite key.

With --cache flag true, kaniko will try to optimize the builds. It will execute all the metadata command like "ENV", "ARG" and update the config.Env to find dependencies between stage.
It discards the updated config changes after the optimize phase.

In this PR, I added a change

1. remove builds args seeded to composite cache key.
2. Replace the command string with any ARG/ENV command which got executed before.

This will ensure, only layers which actually use the ARG will change if ARG value changes.

Added tests for the same.
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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.

`--build-args` will now be added to layer cache key only if it is used in the dockerfile command for that layer.

[cvgw]

Would you mind expanding the Test_stageBuilder_build tests to cover this change by asserting that the cache key received by the mock only contain the correct values. Thanks

[cvgw]

Few questions and more tests

[tejal29]

Answering comments

[tejal29]

@cvgw Please take another look!