Support reading files with non utf8 encoding

[coffee-cup]

Is there an existing issue for this?

• [X] I have searched the existing issues

Describe the bug

If a file (such as requirements.txt) is not in a valid utf8 encoding, we should still support reading it

To reproduce

No response

Expected behavior

No response

Environment

Nixpacks v0.14.0

[Miloas]

For requirements.txt, we should be consistant with pip spec. https://pip.pypa.io/en/stable/reference/requirements-file-format/#encoding

Requirements files are utf-8 encoding by default and also support PEP 263 style comments to change the encoding (i.e. # -- coding: --).