Gentoo Packaging: running ar failed: exec: "ar": executable file not found in $PATH

I'm having a bit of trouble tracking down this issue, so I'm reporting this upstream in the hopes that someone with more experience with Go knows how to fix it.

Downstream bug report is here: https://bugs.gentoo.org/814017

The problem is that something somewhere is calling "ar" directly instead of respecting the user setting in the AR variable. Resulting in compile failure for users who are using a different setting for AR then the default "ar":

/usr/lib/go/pkg/tool/linux_amd64/link: running ar failed: exec: "ar": executable file not found in $PATH

I have grep'ed through the sources, but I cannot find exactly what is calling "ar" and where it is.

See also the build log copied from the downstream bug report: build.log