kleaf: Add p4root symlink into conftest group

The execution of conftest currently relies on the GNU toolchain located
in the P4ROOT path. However, since kleaf is a hermetic build, the P4ROOT
environment variable is cleaned up during the kleaf setup. To address
this, we create a symlink to connect the original P4ROOT path to the
kleaf build path.

Bug 4344670

Change-Id: I7877fd5d5543f73001dd1a017dfd0cc552802658
Signed-off-by: Jian-Min Liu <jianminl@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/c/linux-nv-oot/+/3161994
Reviewed-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: svcacv <svcacv@nvidia.com>
GVS: buildbot_gerritrpt <buildbot_gerritrpt@nvidia.com>

scripts/conftest/BUILD.bazel

@@ -10,5 +10,7 @@ filegroup(
     name = "conftest_headers",
     srcs = glob(["**"],
        allow_empty = False,
-    ),
+    ) + [
+       "//nvidia-build/kleaf:p4root_symlink",
+    ],
 )