sandbox: stop mounting new filesystem on /tmp

Mounting new, empty filesystem on /tmp makes impossible to use mat2 for manipulating files stored there. Especially it breaks running tests while creating package and using /tmp as temporary builddir which is common setup in Arch Linux:
https://aur.archlinux.org/packages/mat2/#comment-721221

libmat2/bubblewrap.py

@@ -53,7 +53,6 @@ def _get_bwrap_args(tempdir: str,
         ['--dev', '/dev',
          '--proc', '/proc',
          '--chdir', cwd,
-         '--tmpfs', '/tmp',
          '--unshare-user-try',
          '--unshare-ipc',
          '--unshare-pid',