Too long paths on Windows

[mosteo]

See alire-project/alire-index#1227 (comment)

In summary: between the root location of the cache, the longish build hashes, and a crate containing long paths, we've hit (once for now) some kind of limit in Windows path lengths.

GitHub runners have long paths enabled, but git long paths are disabled because of collateral issues.

Things to study:

• What's the situation if long paths on Windows itself are not enabled? Which is the most typical default? Are there reasons not to recommend enabling them?
• Do git long paths work with Alire (in particular with our use of msys2)?
• Do \\? prefixed paths help? Should we use them in our syncing?
• Are tar/zip affected in some way, for such origins?