Execute `mach build` from servo's root directory.

slave/build.py

@@ -258,8 +258,16 @@ class ServoBuilder(Builder):
         return os.path.join(self.objdir(), 'release', 'servo')
 
     def make(self):
-        args = [os.path.join('.', self.folder, 'mach'), 'build' ,'--release']
-        Run(args, self.env.get())
+        # Remember cwd
+        cwd = os.getcwd()
+        # cd into servo's root directory. Requirement of 'mach build'
+        os.chdir(self.folder)
+        try:
+            args = [os.path.join('.', 'mach'), 'build' ,'--release']
+            Run(args, self.env.get())
+        finally:
+            # Go back to the cwd
+            os.chdir(cwd)
 
 def getBuilder(config, path):
     # fingerprint the known builders