None is appended to $GITHUB_ENV

[cygniv404]

Describe the bug

When writing a single valid KEY=VALUE line to $GITHUB_ENV in a step, the file ends up with an additional unexpected None appended at the end.
This corrupts the env file, and the runner fails the step with:

Unable to process file command 'env' successfully.
Invalid format 'None'

The $GITHUB_ENV file starts empty at the beginning of the step.
We use either echo or a mv of a temp file to write image_tag=b845a92, and cat $GITHUB_ENV immediately after the write shows both:

image_tag=b845a92
None

The string None is not coming from our scripts — it seems added after our write but before the runner parses it.

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To Reproduce

Steps to reproduce the behavior:

1. Run this minimal job:

jobs:
test:
runs-on: ubuntu-latest
steps:
- name: Set image_tag
run: |
echo "GITHUB_ENV before:"
cat "$GITHUB_ENV" || echo "(empty)"

      image_tag="b845a92"
      tmp=$(mktemp)
      echo "image_tag=$image_tag" > "$tmp"
      mv "$tmp" "$GITHUB_ENV"

      echo "GITHUB_ENV after:"
      cat "$GITHUB_ENV"

2. Observe the output of the final cat shows:

image_tag=b845a92
None

3. The step then fails with:

Unable to process file command 'env' successfully.
Invalid format 'None'

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Expected behavior

Only the valid image_tag=b845a92 should appear in $GITHUB_ENV after writing.
No additional None line should appear in the file.
The step should succeed.

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Runner Version and Platform

Hosted runner on GitHub Actions (ubuntu-latest)

Likely runner version >= 2.316.0 (current hosted runner version at time of writing)

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What's not working?

Writing to $GITHUB_ENV produces correct content (image_tag=b845a92) but the runner sees an additional None line and fails parsing it.

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Job Log Output

GITHUB_ENV before:
(empty)

Resolved image tag: b845a92

GITHUB_ENV after:
image_tag=b845a92
None

Unable to process file command 'env' successfully.
Invalid format 'None'