What steps will reproduce the problem?
go install cmd/go/ # note the extra slash

What is the expected output?
Either give a error (unlikely) or build & install cmd/go successfully.

What do you see instead?
build output "/Users/minux/work/go/bin" already exists and is a directory

Which compiler are you using (5g, 6g, 8g, gccgo)?
6g/8g

Which operating system are you using?
Mac OS X.

Which revision are you using?  (hg identify)
04a64046570e  tip

Please provide any additional information below.
go install -x cmd/go/ output:
WORK=/var/folders/sV/sVMbZDf5EiSjQyuDVzTxn++++TI/-Tmp-/go-build755263920
mkdir -p $WORK/cmd/go/_obj/
cd $GOROOT/src/cmd/go
$GOROOT/pkg/tool/darwin_amd64/6g -o $WORK/cmd/go/_obj/_go_.6 -p cmd/go/ -I $WORK
./build.go ./clean.go ./fix.go ./fmt.go ./get.go ./help.go ./http.go ./list.go ./main.go
./pkg.go ./run.go ./test.go ./testflag.go ./tool.go ./vcs.go ./version.go ./vet.go
$GOROOT/pkg/tool/darwin_amd64/pack grc $WORK/cmd/go/.a $WORK/cmd/go/_obj/_go_.6
$GOROOT/pkg/tool/darwin_amd64/6l -o $WORK/cmd/go/_obj/a.out -L $WORK $WORK/cmd/go/.a
mkdir -p $GOROOT/
cp $WORK/cmd/go/_obj/a.out $GOBIN
build output "/Users/minux/work/go/bin" already exists and is a directory