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 <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

cloudflare/go,vsekhar/elastic-go,golang/go,josharian/go,josharian/go,josharian/go,polyverse-security/go,codestation/go,golang/go,vsekhar/elastic-go,CAFxX/go,golang/go,linux-on-ibm-z/go,CAFxX/go,cloudflare/go,cloudflare/go,CAFxX/go,polyverse-security/go,vsekhar/elastic-go,vsekhar/elastic-go,nawawi/go,polyverse-security/go,codestation/go,Cofyc/go,vsekhar/elastic-go,vsekhar/elastic-go,polyverse-security/go,ImJasonH/go,codestation/go,derekmarcotte/go,polyverse-security/go,ImJasonH/go,CAFxX/go,ImJasonH/go,derekmarcotte/go,golang/go,akutz/go,vsekhar/elastic-go,polyverse-security/go,akutz/go,christopher-henderson/Go,josharian/go,josharian/go,cloudflare/go,derekmarcotte/go,polyverse-security/go,akutz/go,nawawi/go,linux-on-ibm-z/go,ImJasonH/go,linux-on-ibm-z/go,CAFxX/go,nawawi/go,golang/go,golang/go,akutz/go,akutz/go,Cofyc/go,ImJasonH/go,derekmarcotte/go,cloudflare/go,cloudflare/go,Cofyc/go,nawawi/go,Cofyc/go,linux-on-ibm-z/go,derekmarcotte/go,akutz/go,Cofyc/go,codestation/go,CAFxX/go,ImJasonH/go,codestation/go,codestation/go,cloudflare/go,ImJasonH/go,codestation/go,Cofyc/go,CAFxX/go,cloudflare/go,josharian/go,akutz/go,ImJasonH/go,Cofyc/go,linux-on-ibm-z/go,linux-on-ibm-z/go,josharian/go,codestation/go,nawawi/go,golang/go,nawawi/go,akutz/go,nawawi/go,CAFxX/go,Cofyc/go,derekmarcotte/go,linux-on-ibm-z/go,linux-on-ibm-z/go,derekmarcotte/go,polyverse-security/go,nawawi/go,derekmarcotte/go,golang/go,vsekhar/elastic-go,josharian/go