proxy: Fix archiver for public links

Allows /archiver to be used the "public-token" auth middleware. The
archiver is a bit of a special case, because it can be uses in several
ways: using 'normal' authentication (basic, oidc), using signed-urls or
using sharetokens. As only the "sharetoken" part is handled by the
"PublicShareAuth" middleware, we needed to special-case it a bit.
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f