x/tools/gopls: add fallback to default GOPATH if missing
x/tools/gopls: add fallback to default GOPATH if missing

The "Organize imports" code action uses internal/imports that needs a
valid GOPATH set. Since Go 1.8 setting GOPATH manually is not required,
and if it isn't set gopls will sometimes fail to properly import
packages.

This CL sets GOPATH to the default if the env var GOPATH isn't set.

Fixes golang/go#33918.

Change-Id: Ib63a26a801e15af730197999de4d1d4901694a30
Reviewed-on: https://go-review.googlesource.com/c/tools/+/191600
Run-TryBot: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools