Replace deprecated command with environment file

[jongwooo]

Description

Resolve #7920

Update .github/workflows/build.yml to use environment file instead of deprecated set-output command.
For more information, see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

I found the workflow file that use set-output command through the following command:

$ find . -name '*.yml' -o -name '*.yaml' | xargs egrep '\bset-output\b'

AS-IS

run: echo "::set-output name=hashes::$(sha256sum Linux.flatc.binary.${{ matrix.cxx }}.zip | base64 -w0)"

TO-BE

run: echo "hashes=$(sha256sum Linux.flatc.binary.${{ matrix.cxx }}.zip | base64 -w0)" >> $GITHUB_OUTPUT