cmd/go: disable tool version check during bootstrap
    
    The check that the go tool version matched the go compiler version was
    too aggressive and didn't cover the bootstrapping case with make.bash.
    
    We never noticed because we never had a VERSION file in place.
    
    Repro:
    
    $ echo "go1.9beta1" > $GOROOT/VERSION
    $ cd $GOROOT/src
    $ ./make.bash
    
    No test, because x/build/cmd/release catches it.
    
    Updates #19064
    Fixes #20674
    
    Change-Id: Ibdd7a92377f4cc77d71ed548f02d48bde6550f67
    Reviewed-on: https://go-review.googlesource.com/45778
    Reviewed-by: Chris Broadfoot <cbro@golang.org>