Help: Add release note for ExternalProject's environment modification options

This was left out of commit e301cbffcc (ExternalProject: Set environment
variables, 2025-04-09).

Help/release/dev/ExternalProject.rst

@@ -0,0 +1,6 @@
+ExternalProject
+---------------
+
+* The :module:`ExternalProject` module's commands :command:`ExternalProject_Add`
+  and :command:`ExternalProject_Add_Step` now provide options to set
+  environment variables on the configure, build, install, and test steps.