Use tempfiles for disk image preparation

Previously these intermediates were placed in the build directory but
immediately deleted. Unless we are going to make them easily available
for inspection (hopefully not often required), they should just go in
/tmp.

bin/build

@@ -18,6 +18,7 @@ import shlex
 import shutil
 import subprocess
 import sys
+import tempfile
 
 
 
@@ -30,8 +31,8 @@ parser.add_argument('passthru', metavar='ARG', nargs='+', help='Build script arg
 config = parser.parse_args()
 
 config.worktree = os.getcwd()
-config.tmpdir = path.join(config.worktree, '.dmg')
-config.hfsimg = path.join(config.worktree, '.dmg.dmg')
+config.tmpdir = tempfile.mkdtemp()
+handle, config.hfsimg = tempfile.mkstemp(suffix='.dmg')
 config.vmac = open(path.join(path.dirname(__file__), 'vmac_path.conf')).read().rstrip('\n')