Scope build not documented in VirtualRunEnv generator

https://docs.conan.io/2/reference/tools/env/virtualrunenv.html

I was not aware there was a "build" scope for this generator. After talking with @franramirez688 to fix an issue with a CCI recipe, this turned out to be the fix.

I would like to find more information there about the "scopes", what does it mean, and what is the expected behavior.

Start with the linked VirtualRunEnv generator page and read its existing documentation around generator scopes. Verify how the build scope is described and what behavior users should expect, then update that page with the scope information and confirm the rendered documentation explains it clearly.

Written by the indexing model from the issue text.