Mount a new tmpfs on /tmp and drop all capabilities

This mounts a new tmpfs on /tmp so any files residing there would be hidden
from the sandbox. Many programs store some files in there that might be useful
to an attacker.  It also drops all capabilities incase it is ever run with
extra capabilities for whatever reason.

libmat2/subprocess.py

@@ -51,6 +51,7 @@ def _get_bwrap_args(tempdir: str,
         ['--dev', '/dev',
          '--proc', '/proc',
          '--chdir', cwd,
+         '--tmpfs', '/tmp',
          '--unshare-user-try',
          '--unshare-ipc',
          '--unshare-pid',
@@ -58,6 +59,7 @@ def _get_bwrap_args(tempdir: str,
          '--unshare-uts',
          '--unshare-cgroup-try',
          '--new-session',
+         '--cap-drop', 'all',
          # XXX: enable --die-with-parent once all supported platforms have
          # a bubblewrap recent enough to support it.
          # '--die-with-parent',