Fix bubblewrap

On some machines (like mine), `/proc` has to be mounted.  Also, since
sandboxing with bubblewrap is best effort and assumes that an attacker doesn't
have control outside of the file to clean, it's safe to __try__ to enable some
bubblewrap features, and to silently fail otherwise.

libmat2/subprocess.py

@@ -49,8 +49,14 @@ def _get_bwrap_args(tempdir: str,
 
     args = ro_bind_args + \
         ['--dev', '/dev',
+         '--proc', '/proc',
          '--chdir', cwd,
-         '--unshare-all',
+         '--unshare-user-try',
+         '--unshare-ipc',
+         '--unshare-pid',
+         '--unshare-net',
+         '--unshare-uts',
+         '--unshare-cgroup-try',
          '--new-session',
          # XXX: enable --die-with-parent once all supported platforms have
          # a bubblewrap recent enough to support it.