Fixed incorrect custom compile commands from compile_pip_requirements

[UebelAndre]

PR Checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

If a compile_pip_requirements target is setup in a separate package than where the requirements_in file is located, then an incorrect custom compile command is written into the requirements_txt file.

Issue Number: N/A

What is the new behavior?

This change fixes that so no matter where the compile_pip_requirements target is defined, it will always correctly be reflected in the generated files when running the *.update target.

Does this PR introduce a breaking change?

• Yes
• No

Other information