Kokoro: Check lint and doxygen

Move the doxygen CMake output from `out/docs` to `<build>/docs`
`out/docs` requires a GN build or manual directory creation.

Fixed: tint:791
Change-Id: If072f962f444a2d60c368d7c38fd1bb4c5b97caf
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/50309
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: David Neto <dneto@google.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0c9e443..8eb4e45 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -124,7 +124,9 @@
   find_package(Doxygen)
   if(DOXYGEN_FOUND)
     add_custom_target(tint-docs ALL
-        COMMAND ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/Doxyfile
+        COMMAND ${CMAKE_COMMAND}
+          -E env "DOXYGEN_OUTPUT_DIRECTORY=${CMAKE_BINARY_DIR}/docs"
+          ${DOXYGEN_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/Doxyfile
         WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
         COMMENT "Generating API documentation"
         VERBATIM)