Make process improvements

[gouenji-shuuya]

• Move to bash as that was implicitly expected (ref. #vidyut on Discord)
• Some refactoring.
• Sub-make is correctly called when using make create_all_data.
• Use -jnproc in make.
• Ignore venv in git.

[akprasad]

Thank you for this! Forgive the late look.