Add rustup-installed binaries to $PATH on Mac

I missed this in #762.
servo · bors-servo · Jan 8, 2018 · Jan 8, 2018 · Jan 8, 2018 · 730c0bc42fbbaf00bcec762b64148364574e2034
commit 730c0bc42fbbaf00bcec762b64148364574e2034
diff --git a/buildbot/master/files/config/environments.py b/buildbot/master/files/config/environments.py
@@ -78,6 +78,14 @@ def without(self, to_unset):
     'SERVO_CACHE_DIR': '/Users/servo/.servo',
     'OPENSSL_INCLUDE_DIR': '/usr/local/opt/openssl/include',
     'OPENSSL_LIB_DIR': '/usr/local/opt/openssl/lib',
+    'PATH': ':'.join([
+        '{{ common.servo_home }}/.cargo/bin',
+        '/usr/local/bin',
+        '/usr/bin',
+        '/bin',
+        '/usr/sbin',
+        '/sbin',
+    ]),
 })