Don't query GitHub username and use dummy credentials when committing

[andreasnoack]

This is a followup to #1304 since there were two more authentications that relied on querying the potentially non-existent GitHub username.

The second commit might not be the right approach but it allowed me to build new binaries while authenticating as a GitHub app. The issue is that

BinaryBuilder.jl/src/AutoBuild.jl

Line 1589 in ad70f55

commit = LibGit2.commit(wrapper_repo, "$(name)_jll build $(build_version)")

ends up querying username information from the local git repo when calling https://github.com/JuliaLang/julia/blob/187e8c2222878c68b2afc9295ab8dc61773bd7f2/stdlib/LibGit2/src/signature.jl#L66-L72.

[andreasnoack]

After discussions with @DilumAluthge, I've dropped the second commit that used dummy credentials. Instead, the user should set the git default user because building the binaries.

[DilumAluthge]

Thanks Elliot!