create the linux chroots under /var/chroot by default

This is a better location and more likely to have free disk space,
as /opt might be mounted with insufficient disk space (see #1713)

scripts/build.py

@@ -424,8 +424,8 @@ def build_setup_schroot(config, basedir):
     chroot = config[1+config.rindex('-'):]
     for arch in ARCH:
         print '******************* %s-%s' % (chroot, arch)
-        base_dir = os.environ.get('WKHTMLTOX_CHROOT') or '/opt/wkhtmltopdf-build'
-        root_dir = os.path.join(base_dir, '%s-%s' % (chroot, arch))
+        base_dir = os.environ.get('WKHTMLTOX_CHROOT') or '/var/chroot'
+        root_dir = os.path.join(base_dir, 'wkhtmltopdf-%s-%s' % (chroot, arch))
         rmdir(root_dir)
         mkdir_p(root_dir)
         for command in CHROOT_SETUP[chroot]: