Revert "open_then_run: open directory as path fd"

This reverts commit 9646ebf8a7a2d8a78adc595a894c24776c6d1892.

fd_server actually expects a directory FD but not path FD, especially
since it may not have the privilege to open the directory by path.
Passing the path FD doesn't really help anyway, since fd_server could
simply create one by itself.

See also commit 168040291ca4cb18996f93582336825388c59680.

Test: atest AuthFsHostTest

Change-Id: I0f28a3990f514723a143d38ea89e5dc06f442a9b
diff --git a/authfs/tests/Android.bp b/authfs/tests/Android.bp
index 92fa428..6b3a474 100644
--- a/authfs/tests/Android.bp
+++ b/authfs/tests/Android.bp
@@ -38,10 +38,10 @@
     rustlibs: [
         "libandroid_logger",
         "libanyhow",
-        "liblibc",
         "libclap",
         "libcommand_fds",
         "liblog_rust",
+        "libnix",
     ],
     test_suites: ["general-tests"],
     test_harness: false,