Fix appvyeor's perl.exe path

The one which is in PATH by default is MinGW perl, which uses Unix paths.
This perl does not work with mtr.

appveyor.yml

@@ -14,7 +14,7 @@ configuration: RelWithDebInfo
 platform: x64
 
 test_script:
-  - set PATH=%PATH%;C:\Program Files (x86)\Windows Kits\10\Debuggers\x64
+  - set PATH=C:\Strawberry\perl\bin;%PATH%;C:\Program Files (x86)\Windows Kits\10\Debuggers\x64
   - cd %APPVEYOR_BUILD_FOLDER%\win_build\mysql-test
   - perl mysql-test-run.pl --force --max-test-fail=10 --parallel=4 --testcase-timeout=10 --skip-test-list=unstable-tests --suite=main