Tests: Match expected environment modification more consistently

Revert the regex change from commit 01147454e7 (FASTBuild: Add
generator, 2025-07-30) and revise the build output to match anyway.
The actual problem was that build and install steps are driven through
a nested `cmake --build`, and `fbuild` prefixes the first line of build
output.  Add an innocuous first line so we can match the intended lines
consistently across generators.

Tests/RunCMake/ExternalProject/EnvVars-build-stdout.txt

@@ -27,6 +27,6 @@
  *-- Separator: ,
  *-- List: 7,8,9
 .*(Performing build step for 'DefaultCommand'|DefaultCommand-build).*
-.*-- Stage: build
+ *-- Stage: build
  *-- Separator: ,
  *-- List: 7,8,9,10

Tests/RunCMake/ExternalProject/EnvVars/CMakeLists.txt

@@ -6,7 +6,10 @@ message(STATUS "${VARIABLE}: $ENV{${VARIABLE}}")
 message(STATUS "Separator: $ENV{ListSeparator}")
 message(STATUS "List: $ENV{ListVar}")
 
-add_custom_target(EchoEnvVars ALL COMMAND
-  "${CMAKE_COMMAND}" -P "${CMAKE_CURRENT_SOURCE_DIR}/EchoVar.cmake")
+add_custom_target(EchoEnvVars ALL COMMENT "Build environment..."
+  COMMAND "${CMAKE_COMMAND}" -P "${CMAKE_CURRENT_SOURCE_DIR}/EchoVar.cmake")
 
 install(SCRIPT "${CMAKE_CURRENT_SOURCE_DIR}/EchoVar.cmake")
+
+# Avoid repeating always-out-of-date EchoEnvVars during install step.
+set(CMAKE_SKIP_INSTALL_ALL_DEPENDENCY 1)