nsis: add missing file.close()

[GerardoAllende]

Summary

Many changes to files in python do not go into effect until after the file is closed, so if your script edits, leaves open, and reads a file, it won't see the edits.

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.)

[reingart]

Thanks!
LGTM, seems safe and test are passing