Update test-pr action from set-env to $GITHUB_ENV file

See https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

.github/workflows/test-pr.yml

@@ -56,7 +56,7 @@ jobs:
           # enable symlinks on Windows (https://git-scm.com/docs/git-config#Documentation/git-config.txt-coresymlinks)
           git config --global core.symlinks true
           # also, make sure Windows symlinks are *real* symlinks (https://github.com/git-for-windows/git/pull/156)
-          echo '::set-env name=MSYS::winsymlinks:nativestrict'
+          echo 'MSYS=winsymlinks:nativestrict' >> "$GITHUB_ENV"
           # https://github.com/docker-library/bashbrew/blob/a40a54d4d81b9fd2e39b4d7ba3fe203e8b022a67/scripts/github-actions/generate.sh#L146-L149
         if: runner.os == 'Windows'
       - uses: actions/checkout@v2