properly skip non-build flags for 'go list'

If the flags list included ["-o" "binary"], we would properly skip "-o",
but we wouldn't skip "binary".

Thus, 'go list' would receive "binary" as the first argument, and assume
that's the first parameter and the end of the flags.

And add a unit test case.

Fixes #82, again.

main.go

@@ -1030,7 +1030,7 @@ func filterBuildFlags(flags []string) (filtered []string) {
 			continue
 		}
 		// "-name value", so the next arg is part of this flag.
-		if i++; i < len(flags) {
+		if i++; buildFlag && i < len(flags) {
 			filtered = append(filtered, flags[i])
 		}
 	}

main_test.go

@@ -392,6 +392,11 @@ func TestFilterBuildFlags(t *testing.T) {
 			[]string{"-short", "-tags", "foo", "-mod=readonly", "-json"},
 			[]string{"-tags", "foo", "-mod=readonly"},
 		},
+		{
+			"NonBinarySkipped",
+			[]string{"-o", "binary", "-tags", "foo"},
+			[]string{"-tags", "foo"},
+		},
 	}
 	for _, test := range tests {
 		test := test