ExternalProject: Fix environment modification ops named with underscores

Extend commit e301cbffcc (ExternalProject: Set environment variables,
2025-04-09, v4.2.0-rc1~480^2) and commit 9cb590b843 (ExternalProject:
Fix lists and empty commands with environment modification, 2025-09-11,
v4.2.0-rc1~9^2) to match `_` in environment modification operation
names.

Fixes: #27409