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

Modules/ExternalProject.cmake

@@ -2285,7 +2285,7 @@ function(ExternalProject_Add_Step name step)
   if(environment)
     set(env_args "")
     foreach(env_mod IN LISTS environment)
-      if(env_mod MATCHES [[^([^=:]+)=([a-z]+):(.*)$]])
+      if(env_mod MATCHES [[^([^=:]+)=([a-z_]+):(.*)$]])
         set(_value "${CMAKE_MATCH_3}")
         # Replace the separator only in the value in case it is `:`.
         if(sep)

Tests/RunCMake/ExternalProject/EnvVars.cmake

@@ -67,7 +67,7 @@ ExternalProject_Add(DefaultCommand
   CMAKE_ARGS
     -DVARIABLE=ConfigVar
   CONFIGURE_ENVIRONMENT_MODIFICATION
-    ConfigVar=set:config
+    ConfigVar=string_append:fig # RunCMakeTest.cmake sets to "con"
     ListVar=set:7,8,9
     ListSeparator=set:,
   BUILD_ENVIRONMENT_MODIFICATION

Tests/RunCMake/ExternalProject/RunCMakeTest.cmake

@@ -261,6 +261,8 @@ if(GIT_EXECUTABLE)
 endif()
 
 set(RunCMake_TEST_OUTPUT_MERGE 1)
+set(ENV{ConfigVar} "con")
 __ep_test_with_build(EnvVars)
+unset(ENV{ConfigVar})
 unset(RunCMake_TEST_OUTPUT_MERGE)
 run_cmake(InvalidEnvModification)