Allow custom chunk size for regex calls on Windows

[adriendelsalle]

allow customization using env var to cover corner cases with many nested folders

Description

Fix build error on Windows, when paths are too long, or package contains too many files, by allowing users to set the chunk size for regex operation on Windows using environment variable CONDA_BLD_REGEX_CHUNK_SIZE.

I don't know what's the best way to pass this configuration value.
Maybe we could also add a try/catch around the regex call to give a hint if it fails.

Fixes #5122

Checklist - did you ...

• Add a file to the news directory (using the template) for the next release's release notes?
• Add / update necessary tests?
• Add / update outdated documentation?

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[adriendelsalle]

@dbast ping, is it possible to review this small PR?

[jacobwilliams]

• Would there be any harm for allowing it on all platforms?
• I don't think CONDA_BLD_REGEX_CHUNK_SIZE is a good name for this variable. It's really about the max command line length for the os.
• I made some more comments in the ticket (FileNotFoundError: [WinError 206] The filename or extension is too long #5122). Really I think the code is fundamentally broken and should be fixed (I might try to work of PR for my suggestions). But I do think a fallback option to the env var like you have a still a good idea.