Avoid using temporary files when overwriting formatted files to preserve file permissions

[sile]

Fixes #103

Copilot Summary

This pull request includes several changes to the Cargo.toml and src/main.rs files, focusing on dependency updates and code simplification. The most important changes include removing the tempfile dependency and simplifying the overwrite function.

Dependency updates:

• Cargo.toml: Removed the tempfile dependency.

Code simplification:

• src/main.rs: Removed the tempfile usage in the overwrite function and replaced it with std::fs::write for writing text to a file.
• src/main.rs: Removed the unused import std::io::Write.