cmd/go/internal/envcmd: report PKG_CONFIG after the CGO group

cmd/go/internal/envcmd: report PKG_CONFIG after the CGO group

Before the change, `go env` reports PKG_CONFIG in between the
CGO env group:

    GOARCH="amd64"
    GOBIN=""
    GOEXE=""
    GOHOSTARCH="amd64"
    GOHOSTOS="darwin"
    GOOS="darwin"
    GOPATH="/Users/jbd"
    GORACE=""
    GOROOT="/Users/jbd/go"
    GOTOOLDIR="/Users/jbd/go/pkg/tool/darwin_amd64"
    GCCGO="gccgo"
    CC="clang"
    GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/lq/qcn67khn4_1b41_g48x3zchh005d21/T/go-build184491598=/tmp/go-build -gno-record-gcc-switches -fno-common"
    CXX="clang++"
    CGO_ENABLED="1"
    PKG_CONFIG="pkg-config"
    CGO_CFLAGS="-g -O2"
    CGO_CPPFLAGS=""
    CGO_CXXFLAGS="-g -O2"
    CGO_FFLAGS="-g -O2"
    CGO_LDFLAGS="-g -O2"

The change makes PKG_CONFIG to be reported as the final item,
and not breaking the CGO_* group apart.

Change-Id: I1e7ed6bdec83009ff118f85c9f0f7b78a67fdd76
Reviewed-on: https://go-review.googlesource.com/37228
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>