lib/envflag: use flag.Set for setting the flags from env vars

This should make visible the set flags at flag.Visit(), which is used later for logging
and exporting the `is_set` label for these flags at /metrics page

lib/envflag/envflag.go

@@ -40,7 +40,7 @@ func Parse() {
 		// Get flag value from environment var.
 		fname := getEnvFlagName(f.Name)
 		if v, ok := os.LookupEnv(fname); ok {
-			if err := f.Value.Set(v); err != nil {
+			if err := flag.Set(f.Name, v); err != nil {
 				// Do not use lib/logger here, since it is uninitialized yet.
 				log.Fatalf("cannot set flag %s to %q, which is read from environment variable %q: %s", f.Name, v, fname, err)
 			}