@ianlancetaylor, at Gophercon somebody wanted to get their program's build tags at runtime and their first guess was to look there. But we don't record the build tags in the binary. (Reportedly there's an existing bug where that feature request was declined, but I can't find it.)
I see. The go/build package is about building other packages. It doesn't tell you anything about your own program. Even go/build.DefaultContext is populated from GOARCH and GOOS in the environment (though naturally the default is the values used to build your program).