Singularity: set share loop devices to yes. (#45846)

https://docs.sylabs.io/guides/main/admin-guide/configfiles.html#loop-devices

shared loop devices: This allows containers running the same image
to share a single loop device. This minimizes loop device usage and
helps optimize kernel cache usage.
Enabling this feature can be particularly useful for large MPI jobs.

var/spack/repos/builtin/packages/singularityce/package.py

@@ -113,6 +113,11 @@ def fix_mksquashfs_path(self):
             "mksquashfs path = {0}".format(squash_path),
             join_path(prefix.etc, self.singularity_name, self.singularity_name + ".conf"),
         )
+        filter_file(
+            r"^shared loop devices = no",
+            "shared loop devices = yes",
+            join_path(prefix.etc, self.singularity_name, self.singularity_name + ".conf"),
+        )
 
     #
     # Assemble a script that fixes the ownership and permissions of several