Bump virtualenv from 20.4.0 to 20.4.2 in /.github/workflows

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.4.0 to 20.4.2.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](pypa/virtualenv@20.4.0...20.4.2)

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

Author: dependabot[bot]

.github/workflows/constraints.txt

@@ -4,4 +4,4 @@ nox==2020.12.31
 nox-poetry==0.8.1
 poetry==1.1.4
 pre-commit==2.10.0
-virtualenv==20.4.0
+virtualenv==20.4.2