gha: replace set-output with append to output file

This patch replaces setting output using `echo set-output` with appending to
`$GITHUB_OUTPUT`. Details can be found
[here](https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/).

.github/workflows/test.yaml

@@ -28,7 +28,7 @@ jobs:
           # the) generated wheels to the date of the commit.
           export SOURCE_DATE_EPOCH=`git show -s --format=%ct`
           python3 -m flit build
-          printf "::set-output name=wheel::%s" dist/*.whl
+          echo wheel=`echo dist/*.whl` >> $GITHUB_OUTPUT
       - name: Upload package artifacts
         uses: actions/upload-artifact@v2
         with:

devtools/_log_gha.py

@@ -1,3 +1,5 @@
+import os
+from pathlib import Path
 from typing import Any
 
 debug = print
@@ -21,5 +23,5 @@ def error(*args: Any) -> None:
 
 
 def set_output(key: str, value: str) -> None:
-    print('::set-output name={}::{}'.format(key, value))
+    Path(os.environ['GITHUB_OUTPUT']).open('a').write(f'{key}={value}\n')
     print('\033[1;35mOUTPUT: {}={}\033[0m'.format(key, value))