Language Servers: Fix go detection

maybe_run only checks the first arugment, which for go modules
would have been `env`. Since `env` exists, it would always attempt
to do the `go install` command, regardless of whether or not go
exists.

This patch moves the env to an exported variable rather than
being passed inline.

install-language-servers.sh

@@ -102,7 +102,8 @@ function install_language_servers() {
 
     # Go
     if want_lang go ;then
-        maybe_run env GO111MODULE=on go install golang.org/x/tools/gopls@latest
+        export GO111MODULE=on
+        maybe_run go install golang.org/x/tools/gopls@latest
     fi
 
     echo ""