Update virtualenv requirement in the all-dependencies group

Updates the requirements on [virtualenv](https://github.com/pypa/virtualenv) to permit the latest version.

Updates `virtualenv` to 20.28.0
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](pypa/virtualenv@20.27.0...20.28.0)

---
updated-dependencies:
- dependency-name: virtualenv
  dependency-type: direct:production
  dependency-group: all-dependencies
...

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

requirements-build.txt

@@ -1,6 +1,6 @@
 # This file is used for running tox, flake8 and building the wheel
 pip~=24.3
-virtualenv~=20.27
+virtualenv~=20.28
 setuptools>=70.0.0 # not directly required, pinned by Snyk to avoid a vulnerability
 logilab-common~=2.0
 m2r~=0.3