meson: avoid using deprecated build_root() method

The meson.build_root() method has been deprecated, so let's switch to
meson.project_build_root(), which usually means the same thing. The case
where it doesn't do the same thing is if Mesa is a subproject to some
other project, but in that case I believe we want the build root of Mesa,
not of the parent project anyway.

Reviewed-by: Eric Engestrom <eric@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20907>

src/amd/compiler/tests/meson.build

@@ -52,7 +52,7 @@ test(
     'aco_tests',
     [aco_tests_files, gen_spirv_files],
     cpp_args : ['-DACO_TEST_SOURCE_DIR="@0@"'.format(meson.current_source_dir()),
-                '-DACO_TEST_BUILD_ROOT="@0@"'.format(meson.build_root()),
+                '-DACO_TEST_BUILD_ROOT="@0@"'.format(meson.project_build_root()),
                 '-DACO_TEST_PYTHON_BIN="@0@"'.format(prog_python.full_path())] +
                cpp_args_aco,
     include_directories : [