go.bbclass: don't use test to check output from ls

* avoids possibly confusing error message in log.do_install like:
  ls: cannot access 'etcd/3.5.7-r0/build/bin/linux_arm64/': No such file or directory

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>