buildserver: make sure cachedir is accessible to VM instance

It seems that KVM/QEMU's 9p sharing is more sensitive to file perms.  If
~/.cache is 0700, then ~/.cache/fdroidserver cannot be mounted in the guest

makebuildserver

@@ -2,6 +2,7 @@
 
 import os
 import requests
+import stat
 import sys
 import subprocess
 import time
@@ -89,6 +90,15 @@ cachedir = config['cachedir']
 if not os.path.exists(cachedir):
     os.makedirs(cachedir, 0o755)
 
+tmp = cachedir
+while tmp != '/':
+    mode = os.stat(tmp).st_mode
+    if not (stat.S_IXUSR & mode and stat.S_IXGRP & mode and stat.S_IXOTH & mode):
+        print('ERROR:', tmp, 'will not be accessible to the VM!  To fix, run:')
+        print('  chmod a+X', tmp)
+        sys.exit(1)
+    tmp = os.path.dirname(tmp)
+
 if config['apt_package_cache']:
     config['aptcachedir'] = cachedir + '/apt/archives'