Use SOURCE_DATE_EPOCH environment variable to set mtime of files in wheel

[zmanji]

Currently running maturin build does not have reproducible wheel output even if the input files have not changed. This can be observed by doing the following:

1. Running maturin build
2. Record the md5sum of the output wheel
3. Running the same maturin build command again without changing any of the python or rust files
4. Observe the md5sum of the output wheel is different

I noticed in my case it's because the mtime timestamp of the files in the wheel have been updated.

It would be great if maturin respected the SOURCE_DATE_EPOCH environment variable like wheel already does:
https://github.com/pypa/wheel/blob/70d8f7f5189fac0dfbf21b83adbd2781d2537c17/src/wheel/wheelfile.py#L26-L31

If this environment variable is set, it would be used to set the mtime of the files in the wheel, making the output reproducible.

Having reproducibility is really handy for me because a build system may skip tasks if the wheel is the same.

I think the change would be as simple as adjusting

maturin/src/module_writer.rs

Lines 237 to 241 in 6e7b02f

	let options = zip::write::FileOptions::default()
	.unix_permissions(permissions)
	.compression_method(compression_method);
	self.zip.start_file(target.clone(), options)?;
	self.zip.write_all(bytes)?;

to explicitly set the mtime.

[messense]

Pull requests are welcome.