Forbid Pkg to try and download artifact from outside world?

I just managed to add an artifact to one of my packages, however while attempting that I had some address issues to my local storage. This resulted in Pkg errors during instantiate.

The error prompted that Pkg tried to download the artifact from https://pkg.julialang.org/artifact/....
Given that my artifacts are limited to a local network, is there a way to restrict that behavior e.g. by adding something about the Platform in the artifact description ?
Also why is Pkg attempting to reach a domain that is not referenced in the Artifacts.toml ?

My Artifacts.toml looks like the following (1 artifact / 1 rule)

[[<myartifactname>]]
arch = "x86_64"
git-tree-sha1 = "<artifacthash>"
os = "windows"

    [[<myartifactname>.download]]
    sha256 = "<artifactdownloadhash>"
    url = "<artifactlocalurl>"
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