Pass context to httpGet

[hryx]

This fixes a bug where the context is getting canceled immediately after making a request, due to defering the cancel() within a helper function. The symptom was that all cached files are truncated to a few KB.

Mirrors should delete truncated cached files after upgrading wrench.

As a follow up, we should also write the response to a temporary file and atomically swap it with the destination afterward. Even with this fix, it is possible to cache a truncated file at a served path when the connection to ziglang.org is dropped.

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