Deprecation warning printed from GHA when running this action

[ararslan]

From AbstractFFTs PR #79 (relevant CI log):

  REPONAME="AbstractFFTs.jl"
  PACKAGENAME=${REPONAME%.jl}
  echo "::set-output name=packagename::$PACKAGENAME"
  if [[ '' == '' ]]; then
    TESTSCRIPT="using ${PACKAGENAME}"
  else
    TESTSCRIPT=
  fi
  echo "::set-output name=testscript::$TESTSCRIPT"
  shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}
Warning: The `set-output` command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-[10](https://github.com/JuliaMath/AbstractFFTs.jl/actions/runs/3395185311/jobs/5644730618#step:8:11)-[11](https://github.com/JuliaMath/AbstractFFTs.jl/actions/runs/3395185311/jobs/5644730618#step:8:12)-github-actions-deprecating-save-state-and-set-output-commands/
Warning: The `set-output` command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

[SaschaMann]

Thanks! #6 should fix these