internal/lsp/cache: copy *packages.Config environment completely

I noticed a race in these logs:
https://build.golang.org/log/ec2915e97319de219284ed022338f2ebc549aff6.

We need to copy the environment and build flags for each config, since
we're treating the config as single use.

Change-Id: I9e717e688def088cb60f2b23b71d731e2b20b259
Reviewed-on: https://go-review.googlesource.com/c/tools/+/244118
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

internal/lsp/cache/snapshot.go

@@ -107,10 +107,10 @@ func (s *snapshot) config(ctx context.Context) *packages.Config {
 	s.view.optionsMu.Unlock()
 
 	cfg := &packages.Config{
-		Env:        env,
-		Dir:        s.view.folder.Filename(),
 		Context:    ctx,
-		BuildFlags: buildFlags,
+		Dir:        s.view.folder.Filename(),
+		Env:        append([]string{}, env...),
+		BuildFlags: append([]string{}, buildFlags...),
 		Mode: packages.NeedName |
 			packages.NeedFiles |
 			packages.NeedCompiledGoFiles |