ci: fix saving artifacts to S3 for FreeBSD

* Add an extra step that should be considered as a workaround for the
  runner agent (ChristopherHX/github-act-runner) that is used on FreeBSD
  machines. Without it, env.ARTIFACT_EXISTS will be undefined for some
  reason.

* In the s3-upload-artifact action change `if: env.ARTIFACT_EXISTS` to
  `if: env.ARTIFACT_EXISTS == 'true'` to be compatible with the FreeBSD
  runner agent.

Follows up tarantool/multivac#116

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