fix: use correct shared library directory for Lake plugin on Windows

This PR fixes the Windows dev build by using `CMAKE_RELATIVE_LIBRARY_OUTPUT_DIRECTORY`
instead of the hardcoded `lib/lean` path for the Lake plugin. On Windows, DLLs must be
placed next to executables in `bin/`, but the plugin path was hardcoded to `lib/lean`.

Closes #13126

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lakefile.toml.in

@@ -77,7 +77,7 @@ globs = ["Lake.*"]
 defaultFacets = ["static", "static.export"]
 # Load the previous stage's lake native code into lake's build process in order to prevent ABI
 # breakages from affecting bootstrapping.
-moreLeanArgs = ["--plugin", "${PREV_STAGE}/lib/lean/libLake_shared${CMAKE_SHARED_LIBRARY_SUFFIX}"]
+moreLeanArgs = ["--plugin", "${PREV_STAGE}/${CMAKE_RELATIVE_LIBRARY_OUTPUT_DIRECTORY}/libLake_shared${CMAKE_SHARED_LIBRARY_SUFFIX}"]
 
 [[lean_lib]]
 name = "LakeMain"