There is a circular dependency between runtime/race and cmd/cgo in -race build.
Everything depends on runtime/race, but runtime/race is a cgo package.
Currently it's resolved by a hack in go tool:

    // If we are not doing a cross-build, then record the binary we'll
    // generate for cgo as a dependency of the build of any package
    // using cgo, to make sure we do not overwrite the binary while
    // a package is using it.  If this is a cross-build, then the cgo we
    // are writing is not the cgo we need to use.
    if goos == runtime.GOOS && goarch == runtime.GOARCH && !buildRace {

We pretend that packages do not depend on cmd/cgo. But as the result we can overwrite
cmd/cgo binary while it is used.

A better solution is to make runtime/race to be not dependent on cmd/cgo.