--kaniko-dir should take precedence over KANIKO_DIR fix

[prima101112]

Fixes #3098

Description

The --kaniko-dir command-line flag is now given precedence over the KANIKO_DIR environment variable if both are set. This change ensures that the specified directory via the command-line flag will always be used for storing Kaniko's intermediate files and cache. This is could achieve by force os.SetEnv the KANIKO_DIR to --kaniko-dir flag.

Submitter Checklist

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

• [ ] Includes unit tests
• [ ] Adds integration tests if needed.

Reviewer Notes

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

Release Notes

The --kaniko-dir command-line flag is now given precedence over the KANIKO_DIR environment variable.

Examples of user facing changes:
No changes

[prima101112]

@JeromeJu