runtime: check the correct environment variable in TestCgoCallbackGC

The test checks RUNTIME_TESTING_SHORT, whereas the test runner
actually set RUNTIME_TEST_SHORT. Check the correct one.

Updates #32023.

Change-Id: Ie8ab00e1f5b8c02112a9aa1ee0e56028185c8a44
Reviewed-on: https://go-review.googlesource.com/c/go/+/373614


Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>