feat(JLLEnvs): try to load local Artifacts.toml if existed

[johnnychen94]

Almost certainly I get network issues when using Clang in China unless I set proxies. This PR works around it so I figured people might want to see this change.

julia> using Clang
ERROR: InitError: Failed to connect to raw.githubusercontent.com port 443: Connection refused while requesting https://raw.githubusercontent.com/JuliaPackaging/BinaryBuilderBase.jl/master/Artifacts.toml

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This commit provides two changes:

• By default, use the stable release version of Artifacts.toml provided by binarybuilderbase. previously it uses the master version, which can sometimes be breaking and troublesome.
• it utilizes the local disk cache and thus pkg server so that we don't need to fetch resources from GitHub.com and thus is more friendly to users behind the firewall.

As a consequence of getting rid of the need to download from remote server, this commit makes @time using Clang much faster: from 1.7s to 0.4s in Julia 1.7, macOS Intel i9-9880H.

[Gnimuc]

I'm considering directly shipping this Artifact.toml in Clang.jl.

[melonedo]

This is very nice! I have always manually redirected that Artifacts.toml to a local file before. Additionally, we can also avoid directly calling the Pkg.download_artifact function instead of Pkg.ensure_artifact_downloaded, which respects pkg mirror.

[johnnychen94]

I'm considering directly shipping this Artifact.toml in Clang.jl.

I've thought about this too, but then I feel it brings more future synchronization/maintenance work to do without CompatHelper-like CI tools. Thus I took this tricky version.

[Gnimuc]

closed in favor of #377