Fix skydb on linux when adb is not in path.

sky/tools/skydb

 #!/usr/bin/env python
 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 from skypy.skyserver import SkyServer
 import argparse
 import json
 import logging
 import os
@@ skipping 19 matching lines @@
 GDB_PORT = 8888
 SKY_SERVER_PORT = 9999
 DEFAULT_URL = "https://raw.githubusercontent.com/domokit/mojo/master/sky/examples/home.sky"
 
 ANDROID_PACKAGE = "org.chromium.mojo.shell"
 ANDROID_ACTIVITY = "%s/.MojoShellActivity" % ANDROID_PACKAGE
 ANDROID_APK_NAME = 'MojoShell.apk'
 
 PID_FILE_PATH = "/tmp/skydb.pids"
 CACHE_LINKS_PATH = '/tmp/mojo_cache_links'
-# TODO(iansf): Fix undefined behavior when you have more than one device attached.
-SYSTEM_LIBS_ROOT_PATH = '/tmp/device_libs/%s' % (subprocess.check_output(['adb', 'get-serialno']).strip())
 
 SRC_ROOT = skypy.paths.Paths('ignored').src_root
 ADB_PATH = os.path.join(SRC_ROOT,
     'third_party/android_tools/sdk/platform-tools/adb')
 
+# TODO(iansf): Fix undefined behavior when you have more than one device attached.
+SYSTEM_LIBS_ROOT_PATH = '/tmp/device_libs/%s' % (subprocess.check_output([ADB_PATH, 'get-serialno']).strip())
+
 
 # FIXME: Move this into mopy.config
 def gn_args_from_build_dir(build_dir):
     gn_cmd = [
         'gn', 'args',
         build_dir,
         '--list', '--short'
     ]
     config = {}
     for line in subprocess.check_output(gn_cmd).strip().split('\n'):
@@ skipping 580 matching lines @@
         load_parser.set_defaults(func=self.load_command)
 
         args = parser.parse_args()
         args.func(args)
 
         self._write_pid_file(PID_FILE_PATH, self.pids)
 
 
 if __name__ == '__main__':
     SkyDebugger().main()