Add PIP_REQUIREMENTS_FILE environment variable to CLI flags

What does this MR do?

Add PIP_REQUIREMENTS_FILE environment variable to allow users to specify a custom pip requirements file.

Prior to that change, flagRequirementsFile and envVarRequirementsFile are declared as constants, and flagRequirementsFile is used to initialize the Finder, but finder.Flags() doesn't return a CLI flag for it.

What are the relevant issue numbers?

gitlab-org/gitlab#324238 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
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• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer