chore: exclude .nox from linting (#139)

* changes without context

        autosynth cannot find the source of changes triggered by earlier changes in this
        repository, or by version upgrades to tools such as linters.

* chore: exclude `.nox` directories from linting

The samples tests create `.nox` directories
with all dependencies installed. These directories
should be excluded from linting.

I've tested this change locally, and it significantly
speeds up linting on my machine.

Source-Author: Tim Swast <swast@google.com>
Source-Date: Tue Dec 22 13:04:04 2020 -0600
Source-Repo: googleapis/synthtool
Source-Sha: 373861061648b5fe5e0ac4f8a38b32d639ee93e4
Source-Link: googleapis/synthtool@3738610

packages/google-cloud-asset/.flake8

@@ -26,6 +26,7 @@ exclude =
   *_pb2.py
 
   # Standard linting exemptions.
+  **/.nox/**
   __pycache__,
   .git,
   *.pyc,

packages/google-cloud-asset/synth.metadata

@@ -19,14 +19,14 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "363fe305e9ce34a6cd53951c6ee5f997094b54ee"
+        "sha": "373861061648b5fe5e0ac4f8a38b32d639ee93e4"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "363fe305e9ce34a6cd53951c6ee5f997094b54ee"
+        "sha": "373861061648b5fe5e0ac4f8a38b32d639ee93e4"
       }
     }
   ],