internal/gocommand: simplify GoVersion invocation

Detecting the go version does not require build flags,
and should not depend on the value of GO111MODULE.

For golang/go#59841.

Change-Id: I74255953985e6d2ad01095eea2b6d72f7d280e3f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/489215
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>

internal/gocommand/version.go

@@ -23,21 +23,11 @@ import (
 func GoVersion(ctx context.Context, inv Invocation, r *Runner) (int, error) {
 	inv.Verb = "list"
 	inv.Args = []string{"-e", "-f", `{{context.ReleaseTags}}`, `--`, `unsafe`}
-	inv.Env = append(append([]string{}, inv.Env...), "GO111MODULE=off")
-	// Unset any unneeded flags, and remove them from BuildFlags, if they're
-	// present.
-	inv.ModFile = ""
+	inv.BuildFlags = nil // This is not a build command.
 	inv.ModFlag = ""
-	var buildFlags []string
-	for _, flag := range inv.BuildFlags {
-		// Flags can be prefixed by one or two dashes.
-		f := strings.TrimPrefix(strings.TrimPrefix(flag, "-"), "-")
-		if strings.HasPrefix(f, "mod=") || strings.HasPrefix(f, "modfile=") {
-			continue
-		}
-		buildFlags = append(buildFlags, flag)
-	}
-	inv.BuildFlags = buildFlags
+	inv.ModFile = ""
+	inv.Env = append(inv.Env[:len(inv.Env):len(inv.Env)], "GO111MODULE=off")
+
 	stdoutBytes, err := r.Run(ctx, inv)
 	if err != nil {
 		return 0, err