Handle permissions better

[supakeen]

Image Builder requires root permissions for many tasks (but not all). This is due to needing root permissions to do any disk mounting.

However, image-builder-cli always needs root permissions due to trying to create a directory in /var/cache.

If we're going down that route then perhaps we should add a geteuid() check and exit earlier. Right now it takes (on my system) 10-15 seconds before we get to the point of error:

user@muja ~ € image-builder build iot-commit
No distro name specified, selecting "fedora-41" based on host, use --distro to overrideWARN[0000] Failed to load consumer certs: no consumer key found 
Traceback (most recent call last):
  File "/usr/bin/osbuild", line 33, in <module>
    sys.exit(load_entry_point('osbuild==138', 'console_scripts', 'osbuild')())
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
  File "/usr/lib/python3.13/site-packages/osbuild/main_cli.py", line 171, in osbuild_cli
    with ObjectStore(args.store) as object_store:
         ~~~~~~~~~~~^^^^^^^^^^^^
  File "/usr/lib/python3.13/site-packages/osbuild/objectstore.py", line 325, in __init__
    os.makedirs(self.store, exist_ok=True)
    ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen os>", line 217, in makedirs
  File "<frozen os>", line 227, in makedirs
PermissionError: [Errno 13] Permission denied: '/var/cache/image-builder'
error: running osbuild failed: exit status 1

Alternatively, we might want to tag the various image types with if they require root permissions and handle the cache differently.