Move AppVeyor version string script to build section

It does not work in the init section for some reason, with python giving

python .\\build_scripts\\append_to_version_file.py
%APPVEYOR_REPO_COMMIT%
python: can't open file '.\\build_scripts\\append_to_version_file.py':
[Errno 2] No such file or directory

.appveyor.yml

@@ -26,9 +26,9 @@ clone_depth: 1
 
 init:
   - set TAG_NAME=%APPVEYOR_REPO_TAG_NAME%
-  - python ./build_scripts/append_to_version_file.py %APPVEYOR_REPO_COMMIT%
 
 build_script:
+  - python ./build_scripts/append_to_version_file.py %APPVEYOR_REPO_COMMIT%
   - python ./build_scripts/run-clang-format.py ./src -r --clang-format-executable "C:\Program Files\LLVM\bin\clang-format.exe" --color always
   - call "build_windows.cmd"