src/cmd-oscontainer: use .format() instead of f-strings

In the EL7 version of the `cosa` image, the `cmd-oscontainer` script
gets mangled to use Python2, so f-strings are not an option here.

src/cmd-oscontainer

@@ -42,7 +42,7 @@ def oscontainer_extract(containers_storage, src, dest,
     if commit is None:
         raise SystemExit("Failed to find label '{}'".format(OSCONTAINER_COMMIT_LABEL))
     iid = inspect['Id']
-    print(f"Preparing to extract cid: {iid}")
+    print("Preparing to extract cid: {}".format(iid))
     # We're not actually going to run the container. The main thing `create` does
     # then for us is "materialize" the merged rootfs, so we can mount it.
     # In theory we shouldn't need --entrypoint=/enoent here, but