cmd/go: update go bug to be more consistent with Github issue template

cmd/go: update go bug to be more consistent with Github issue template

As a result of using go env, the following new environment variables are
shown as part of the env section:

+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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build612849170=/tmp/go-build -gno-record-gcc-switches"

The diff between the web-based template and the result of go bug is now:

+GOROOT/bin/go version: go version devel +478f3a5384 Wed Mar 27 16:21:00 2019 +0000 linux/amd64
+GOROOT/bin/go tool compile -V: compile version devel +478f3a5384 Wed Mar 27 16:21:00 2019 +0000
+uname -sr: Linux 4.15.0-46-generic
+Distributor ID:        Ubuntu
+Description:   Ubuntu 18.04.2 LTS
+Release:       18.04
+Codename:      bionic
+/lib/x86_64-linux-gnu/libc.so.6: GNU C Library (Ubuntu GLIBC 2.27-3ubuntu1) stable release version 2.27.

Fixes #26751

Change-Id: I32baca1c3c06d08068dad0041a43a1f5532bd91e
Reviewed-on: https://go-review.googlesource.com/c/go/+/127495
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>