x/build: configure dashboard builders to test the same configurations that will be shipped in binary releases

[bcmills]

#43232 notes a case in which go test on a package in std fails for a Go toolchain installed from our supported binary distributions, but passes for a Go toolchain built from source.

Even if we fix that issue, we don't currently have a good way to prevent regressions, and it makes me wonder if there are other cases where we're shipping packages and/or tests that work locally but fail for users who use our provided installers.

For the platforms for which we ship official binaries, I think we should have dashboard builders that test the actual bytes that we would ship in a release — either by applying the same transformations that we will apply at release time, or by eliminating differences between the release process and make.bash.

(Related: #29205, #29239.)

CC @golang/release

[gopherbot]

Change https://golang.org/cl/330252 mentions this issue: go/types: in TestStdlib, import from source instead of export data