run_test.sh: Fix bazelisk location detection
run_test.sh: Fix bazelisk location detection

In I4b5a32a15d WCT tests invocation was switched from bazel to
bazelisk, but bazelisk location command was wrong. stderr and
stdout was erroneously redirected to the /dev/null, so that the
actual location of the bazelisk command was lost. Fix the location
detection command by redirecting only stderr but not stdout to the
null device.

Change-Id: I66cb33e0074675e00b0e58bb11413b5151a57533

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