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

Example: https://github.com/lurk-lab/arecibo/pull/352#issuecomment-1984696614

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.

Start with .github/workflows/bench-pr-comment.yml and reproduce the first !benchmark run described in the issue. Inspect how the workflow handles the env file command and its newline or whitespace; done means the first benchmark run succeeds without the reported format error and subsequent runs continue to work.

Written by the indexing model from the issue text.