open_then_run: open directory as path fd

For using the FD with openat(2) or mkdirfd(2), it has to be a path FD.
No need to open readonly since most flags are ignored with O_PATH.

Bug: 203251769
Test: AuthFsHostTest

Change-Id: Ide1cb65f6354c37776174d1177450565616c600e
diff --git a/authfs/tests/Android.bp b/authfs/tests/Android.bp
index fd45e13..88c1ba6 100644
--- a/authfs/tests/Android.bp
+++ b/authfs/tests/Android.bp
@@ -29,10 +29,10 @@
     rustlibs: [
         "libandroid_logger",
         "libanyhow",
+        "liblibc",
         "libclap",
         "libcommand_fds",
         "liblog_rust",
-        "libnix",
     ],
     test_suites: ["general-tests"],
     test_harness: false,