chore(deps): update dependency virtualenv to v20.28.1 (#2101)

Author: renovate-bot

.kokoro/docker/docs/requirements.txt

@@ -66,7 +66,7 @@ tomli==2.2.1 \
  --hash=sha256:ece47d672db52ac607a3d9599a9d48dcb2f2f735c6c2d1f34130085bb12b112a \
  --hash=sha256:f4039b9cbc3048b2416cc57ab3bda989a6fcf9b36cf8937f01a6e731b64f80d7
  # via nox
-virtualenv==20.28.0 \
- --hash=sha256:23eae1b4516ecd610481eda647f3a7c09aea295055337331bb4e6892ecce47b0 \
- --hash=sha256:2c9c3262bb8e7b87ea801d715fae4495e6032450c71d2309be9550e7364049aa
+virtualenv==20.28.1 \
+ --hash=sha256:412773c85d4dab0409b83ec36f7a6499e72eaf08c80e81e9576bca61831c71cb \
+ --hash=sha256:5d34ab240fdb5d21549b76f9e8ff3af28252f5499fb6d6f031adac4e5a8c5329
  # via nox