RPM stage: link /proc/self/fd to /dev/fd

Link "/proc/self/fd" to "/dev/fd" within the tree
to avoid "'/dev/fd/63': No such file or directory" errors

Signed-off-by: Miguel Martín <mmartinv@redhat.com>

stages/org.osbuild.rpm

@@ -320,6 +320,8 @@ def main(tree, inputs, options):
         os.makedirs(target, exist_ok=True)
         mount(source, target, ro=False)
 
+    os.symlink("/proc/self/fd", f"{tree}/dev/fd")
+
     machine_id_created = create_machine_id_if_needed(tree)
 
     ostree_booted = None