runtime: use GOTRACEBACK=system for TestCgoExecSignalMask
runtime: use GOTRACEBACK=system for TestCgoExecSignalMask

Try to get a bit more information to understand #42093.

For #42093

Change-Id: I818feb08d7561151d52eba3e88c418b55b9f9c1e
Reviewed-on: https://go-review.googlesource.com/c/go/+/264018
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

CAFxX/go,nawawi/go,nawawi/go,cloudflare/go,cloudflare/go,nawawi/go,nawawi/go,golang/go,golang/go,cloudflare/go,nawawi/go,golang/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,CAFxX/go,CAFxX/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,golang/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go