ExternalProject: Restore driving install through build system

Revert commit 66b5d51f38 (ExternalProject: Install CMake projects
using 'cmake --install', 2022-09-08, v3.25.0-rc1~150^2).
It changed the ExternalProject install step command from:

    cmake --build <dir> --target install --config <cfg>

to:

    cmake --install <dir> --config <cfg>

The latter command no longer runs the external project build system
during the install step.  We could consider using the commands:

    cmake --build <dir> --target all --config <cfg>
    cmake --install <dir> --config <cfg>

as the install step, but if `CMAKE_SKIP_INSTALL_ALL_DEPENDENCY` is
used in the external project, that can change semantics too.

Revert the original change pending further investigation on other ways
to support its motivating use case.  Add a test covering the
previously-regressed use case.

Fixes: #24567
Issue: #23946

Modules/ExternalProject.cmake

@@ -1840,11 +1840,7 @@ function(_ep_get_build_command
       else()
         set(cmd "${CMAKE_COMMAND}")
       endif()
-      if(step STREQUAL "INSTALL")
-        set(args --install ".")
-      else()
-        set(args --build ".")
-      endif()
+      set(args --build ".")
       get_property(_isMultiConfig GLOBAL PROPERTY GENERATOR_IS_MULTI_CONFIG)
       if(_isMultiConfig)
         if (CMAKE_CFG_INTDIR AND
@@ -1866,6 +1862,9 @@ function(_ep_get_build_command
         endif()
         list(APPEND args --config ${config})
       endif()
+      if(step STREQUAL "INSTALL")
+        list(APPEND args --target install)
+      endif()
       # But for "TEST" drive the project with corresponding "ctest".
       if("x${step}x" STREQUAL "xTESTx")
         string(REGEX REPLACE "^(.*/)cmake([^/]*)$" "\\1ctest\\2" cmd "${cmd}")

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

@@ -0,0 +1,2 @@
+.*build command suppressed
+.*always builds

Tests/RunCMake/ExternalProject/InstallBuilds.cmake

@@ -0,0 +1,7 @@
+include(ExternalProject)
+
+ExternalProject_Add(InstallBuilds
+  SOURCE_DIR ${CMAKE_CURRENT_LIST_DIR}/InstallBuilds
+  DOWNLOAD_COMMAND ""
+  BUILD_COMMAND "${CMAKE_COMMAND}" -E echo "build command suppressed"
+  )

Tests/RunCMake/ExternalProject/InstallBuilds/CMakeLists.txt

@@ -0,0 +1,4 @@
+cmake_minimum_required(VERSION 3.24)
+project(InstallBuilds NONE)
+add_custom_target(always ALL COMMAND ${CMAKE_COMMAND} -E echo "always builds")
+install(CODE "")

Tests/RunCMake/ExternalProject/RunCMakeTest.cmake

@@ -147,6 +147,7 @@ endfunction()
 __ep_test_with_build(MultiCommand)
 
 set(RunCMake_TEST_OUTPUT_MERGE 1)
+__ep_test_with_build(InstallBuilds)
 __ep_test_with_build(PreserveEmptyArgs)
 set(RunCMake_TEST_OUTPUT_MERGE 0)