tools/gopls: use go install instead of go get

Fixes: golang/go#49114
Change-Id: I39c5bee4168862af2aa5a29a96d45cad3e0dd9f1
Reviewed-on: https://go-review.googlesource.com/c/tools/+/358074
Reviewed-by: Robert Findley <[email protected]>
Reviewed-by: Nooras Saba‎ <[email protected]>
Trust: Robert Findley <[email protected]>
Trust: Peter Weinberger <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
gopls-CI: kokoro <[email protected]>

Author: emahiro

gopls/README.md

@@ -36,7 +36,7 @@ directory that is both outside of your `GOPATH` and outside of a module (a temp
 directory is fine), and run:
 
 ```sh
-GO111MODULE=on go get golang.org/x/tools/gopls@latest
+go install golang.org/x/tools/gopls@latest
 ```
 
 **NOTE**: Do not use the `-u` flag, as it will update your dependencies to