nsis: add missing file.close()

[GerardoAllende]

Summary

Missing file.close() can cause errors when file is read before the garbage collector closes the file.

Checklist

• Classes, Variables, function and methods logic ok
• Comments written explaining what the code does
• All python code is PEP8 compliant (run black .)
• No lint issues (run flake8)
• Test coverage with pytest implemented
• Reviewers assigned (at least 1 mentor)

Manual test evidence

(attach command-line examples, execution output & logs, etc.)