[Build] Repair the build for non-static-only static stdlib builds

When SWIFT_BUILD_STATIC_STDLIB=ON, SWIFT_BUILD_DYNAMIC_STDLIB=OFF, and
the sdk being built is not a static-only (e.g. WASI), the build fails
due to duplicate custom command rules against the same output path.
In the case of WASI, the static archive should be copied into lib/swift
by the first lipo target, and then the second lipo target should copy it
into lib/swift_static.

```
CMake Error at cmake/modules/SwiftAddCustomCommandTarget.cmake:144 (add_custom_command):
  Attempt to add a custom rule to output

    /home/build-user/build/buildbot_linux/wasmstdlib-linux-x86_64/lib/swift_static/wasi/libswiftCore.a.rule

  which already has a custom rule.
Call Stack (most recent call first):
  stdlib/cmake/modules/AddSwiftStdlib.cmake:673 (add_custom_command_target)
  stdlib/cmake/modules/AddSwiftStdlib.cmake:2657 (_add_swift_lipo_target)
  stdlib/public/core/CMakeLists.txt:401 (add_swift_target_library)
```

Author: kateinoigakukun

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -2492,10 +2492,10 @@ function(add_swift_target_library name)
             "${SWIFTLIB_DIR}/${library_subdir}/${CMAKE_SHARED_LIBRARY_PREFIX}${name}${CMAKE_SHARED_LIBRARY_SUFFIX}")
         endif()
       else()
-        if(SWIFTLIB_INSTALL_WITH_SHARED)
-          set(lib_dir "${SWIFTLIB_DIR}")
-        else()
+        if(SWIFT_SDK_${sdk}_STATIC_ONLY)
           set(lib_dir "${SWIFTSTATICLIB_DIR}")
+        else()
+          set(lib_dir "${SWIFTLIB_DIR}")
         endif()
 
         if("${sdk}" STREQUAL "WINDOWS")