README: use $GOBIN instead of $GOPATH/bin

[mvdan]

$GOPATH/bin works in the simple cases, but will break if the user
specifies their own $GOBIN, or if their $GOPATH has multiple elements.

This form is also simpler. Even if the user doesn't specify their custom
$GOBIN, 'go env GOBIN' will return the correct default.

[FiloSottile]

Thanks!

[dmitshur]

Even if the user doesn't specify their custom $GOBIN, 'go env GOBIN' will return the correct default.

@mvdan I can't reproduce this. Any ideas what's different about your setup such that you can?

~ $ go version
go version go1.12.6 darwin/amd64
~ $ echo $GOPATH

~ $ echo $GOBIN

~ $ echo $GO111MODULE

~ $ go env GOBIN

~ $ go env GOPATH
/Users/dmitshur/go

go env Output

~ $ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/dmitshur/Library/Caches/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/dmitshur/go"
GOPROXY="https://proxy.golang.org"
GORACE=""
GOROOT="/usr/local/go"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/3m/rg2zm24d1jg40wb48wr0hdjw00jwcj/T/go-build792247979=/tmp/go-build -gno-record-gcc-switches -fno-common"

[mvdan]

You're right, go env GOBIN doesn't print a default; I might have gotten confused. @FiloSottile feel free to revert this commit, though the old $GOPATH/bin approach is still broken for the two reasons I gave originally.

I think the only properly correct way is go list -f {{.Target}} some/main/package, which gives the path that a certain main package would be installed to. That might be overkill for the README - up to you.