ci: replace deprecated ::set-env

Author: bluetech

.github/workflows/main.yml

@@ -165,7 +165,7 @@ jobs:
     - uses: actions/checkout@v2
     - uses: actions/setup-python@v2
     - name: set PY
-      run: echo "::set-env name=PY::$(python -c 'import hashlib, sys;print(hashlib.sha256(sys.version.encode()+sys.executable.encode()).hexdigest())')"
+      run: echo "name=PY::$(python -c 'import hashlib, sys;print(hashlib.sha256(sys.version.encode()+sys.executable.encode()).hexdigest())')" >> $GITHUB_ENV
     - uses: actions/cache@v1
       with:
         path: ~/.cache/pre-commit