Don't cache 4xx/5xx errors permanently for files from stable releases

[0t4u]

Wrench's Zig mirror service currently caches 404 errors for files that are expected to be available, like files from releases (see: hexops/mach#1416, https://github.com/mlugg/setup-zig/issues/33).

This PR checks if the requested file belongs to a stable version and has a version number greater than 0.5.0 (see: https://github.com/mlugg/setup-zig#adding-a-mirror), and prevents caching 4xx/5xx errors for those files, since it is expected that these files should be available later.

As a followup, we should also check if the version of the file being requested corresponds to the version string of the latest Zig master or latest Mach nominated version and not cache 4xx/5xx errors for those as well.

A better approach may be to read the index file and ensure all versions present there won't have their 4xx/5xx errors cached.

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[emidoots]

as-is, this change would open a vulnerability which allows anyone to ask the server to spam ziglang.org with requests for versions that are not expected to exist.

A better solution to this would be to continue caching 4xx/5xx errors, but use a cache expiry duration so that the cached response expires after some time, e.g. 15 minutes

[0t4u]

Please let me know if the latest change addresses your concerns, I have set the expiry at 5 minutes for now, which seems like a good amount of time for files to propagate on the upstream

[0t4u]

@emidoots could you please review again? Thanks

[emidoots]

Will merge once that race condition gets fixed, my bad for being so late on the review here!