ci: fix MERGE_TOOL_DIR on Windows with Meson

On Windows, we execute tests with "--no-bin-wrappers".

a87e427 (ci: speed up Windows phase, 2019-01-29)

Author: pks-t

t/meson.build

@@ -1208,6 +1208,7 @@ endif
 
 test_environment = script_environment
 test_environment.set('GIT_BUILD_DIR', git_build_dir)
+test_environment.set('MERGE_TOOLS_DIR', meson.project_source_root() / 'mergetools')
 
 foreach integration_test : integration_tests
   test(fs.stem(integration_test), shell,