os: don't assume /bin/pwd in test, find it in $PATH

os: don't assume /bin/pwd in test, find it in $PATH

There are several distros now that no longer have /bin.
Instead of assuming /bin/pwd, we will look for it in $PATH.

Fixes #21684.

Change-Id: I61478326500edeadc3c26803990550dad00c7971
Signed-off-by: Ron Minnich <rminnich@gmail.com>
Reviewed-on: https://go-review.googlesource.com/60010
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>