chore(deps): bump virtualenv from 20.26.3 to 20.26.6 in /.kokoro

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.26.3 to 20.26.6.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](pypa/virtualenv@20.26.3...20.26.6)

---
updated-dependencies:
- dependency-name: virtualenv
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

.kokoro/requirements.txt

@@ -517,9 +517,9 @@ urllib3==2.2.2 \
     # via
     #   requests
     #   twine
-virtualenv==20.26.3 \
-    --hash=sha256:4c43a2a236279d9ea36a0d76f98d84bd6ca94ac4e0f4a3b9d46d05e10fea542a \
-    --hash=sha256:8cc4a31139e796e9a7de2cd5cf2489de1217193116a8fd42328f1bd65f434589
+virtualenv==20.26.6 \
+    --hash=sha256:280aede09a2a5c317e409a00102e7077c6432c5a38f0ef938e643805a7ad2c48 \
+    --hash=sha256:7345cc5b25405607a624d8418154577459c3e0277f5466dd79c49d5e492995f2
     # via nox
 wheel==0.43.0 \
     --hash=sha256:465ef92c69fa5c5da2d1cf8ac40559a8c940886afcef87dcf14b9470862f1d85 \