stages/oci-archive: do not include selinux labels

Instead of including SELinux labels for the content layers via the
`--selinux` tar option, make sure selinux labels are not included by
using the `--no-selinux` option.
The inclusion of the labels was a mistake, since they should be
determined by the target system because selinux labels are not
namespaced. On RHEL/Fedora the SELinux label used is something like
`system_u:object_r:container_ro_file_t:s0` for all the files in the
container.
Including the label was leading to permission problems because
the files had a different label on the host and programs inside
the container get `EACCES`, i.e. Permission denied, errors when
accessing files with the different label.
Interestingly this does not happen on Fedora 33 but only on RHEL.
One possibility is that the overlayfs kernel driver in RHEL is
behaving differently on RHEL than on Fedora.

assemblers/org.osbuild.oci-archive

@@ -142,7 +142,7 @@ def blobs_add_layer(blobs: str, tree: str):
 
     command = [
         "tar",
-        "--selinux",
+        "--no-selinux",
         "--acls",
         "--xattrs",
         "-cf", layer_file,

stages/org.osbuild.oci-archive

@@ -165,7 +165,7 @@ def blobs_add_layer(blobs: str, tree: str):
 
     command = [
         "tar",
-        "--selinux",
+        "--no-selinux",
         "--acls",
         "--xattrs",
         "-cf", layer_file,