`bench-pr-comment.yml` fails on the first run

[samuelburnham]

Example: lurk-lang/arecibo#352 (comment)

When run for the first time in a PR, the workflow fails with

Error: Unable to process file command 'env' successfully.
Error: Invalid format '
'

This is likely a newline or whitespace issue to be debugged. However, the workflow runs correctly on subsequent !benchmark runs for some reason.