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/60010Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/os_test.go

@@ -1014,9 +1014,14 @@ func TestStartProcess(t *testing.T) {
 		dir = Getenv("SystemRoot")
 		args = []string{"/c", "cd"}
 	default:
-		cmd = "/bin/pwd"
+		var err error
+		cmd, err = osexec.LookPath("pwd")
+		if err != nil {
+			t.Fatalf("Can't find pwd: %v", err)
+		}
 		dir = "/"
 		args = []string{}
+		t.Logf("Testing with %v", cmd)
 	}
 	cmddir, cmdbase := filepath.Split(cmd)
 	args = append([]string{cmdbase}, args...)