internal/lsp: force LANG setting while difftest
internal/lsp: force LANG setting while difftest

difftest using output of GNU diff, which will be translated based on
LANG settings.

Fixes golang/go#42962

Change-Id: Id4ae22da04077eb7cda7faace306fdbe1836ec60
Reviewed-on: https://go-review.googlesource.com/c/tools/+/274992
Trust: Meng Zhuo <3e6d300a356a3b86bc195677414a9ec5508ca893@golangcn.org>
Run-TryBot: Meng Zhuo <3e6d300a356a3b86bc195677414a9ec5508ca893@golangcn.org>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>

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