jenkins: allow relative POLLY_ROOT paths

jenkins.py

@@ -111,6 +111,7 @@ def run():
   os.makedirs(testing_dir, exist_ok=True)
 
   if os.name == 'nt':
+    # path too long workaround
     hunter_junctions = os.getenv('HUNTER_JUNCTIONS')
     if hunter_junctions:
       temp_dir = tempfile.mkdtemp(dir=hunter_junctions)
@@ -143,7 +144,7 @@ def run():
   else:
     which = 'which'
 
-  polly_root = os.getenv('POLLY_ROOT')
+  polly_root = os.path.abspath(os.getenv('POLLY_ROOT'))
   if polly_root:
     print('Using POLLY_ROOT: {}'.format(polly_root))
     build_script = os.path.join(polly_root, 'bin', 'build.py')