Replace heredoc with printenv

[0x2b3bfa0]

Continuation of #1 (comment)

I have experimented with several variants, but didn't quite like any of them:¹

• Take one, using printenv for both INPUT_RUN and INPUT_ENV and moving the script header to the YAML.
  • I like how the script looks, but moving the script header far from where it's being used doesn't feel completely right.
• Take two, using printenv for both INPUT_RUN and INPUT_ENV, but leaving the "heredoc" for the script header.
  • I don't like this one, as it needs one additional tee invocation, and maintaining the file path in one more place.
• Take three, using printenv only for INPUT_ENV because that "heredoc" was superflous.
  • I like this change, although it breaks the symmetry between INPUT_RUN and INPUT_ENV

Footnotes

1. Reminds me of XKCD 2408. ↩

[0x2b3bfa0]

I originally decided to close this pull request without merging, as none of the variants seemed valuable enough to me. However, I finally decided it was worth considering, and re-opened it.