Modifying the input slice broke the new test for whether gccgo
supports -fgo-importcfg, as the test passed a slice of the argument
slice it was in the process of building.

Fixes #22089

Change-Id: I45444a82673223c46be0c8579da3e31a74c32d73
Reviewed-on: https://go-review.googlesource.com/67191
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>