ci: bump virtualenv from 20.26.5 to 20.26.6 in /.github/workflows (#774)

Author: dependabot[bot]

.github/workflows/constraints.txt

@@ -2,5 +2,5 @@ pip==24.2
 nox==2024.4.15
 nox-poetry==1.0.3
 poetry==1.8.3
-virtualenv==20.26.5
+virtualenv==20.26.6
 toml==0.10.2