Bump virtualenv from 20.26.3 to 20.26.6 in /.github/workflows

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: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

.github/workflows/constraints.txt

@@ -1,4 +1,4 @@
 pip==24.2
 nox==2024.4.15
 nox-poetry==1.0.3
-virtualenv==20.26.3
+virtualenv==20.26.6