Only check text files for "do not submit" string

[mosuem]

To avoid failures such as https://github.com/dart-lang/native/actions/runs/8236607137/job/22523430279?pr=987.

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:firehose	0.8.0	ready to publish	firehose-v0.8.0
package:dart_flutter_team_lints	2.1.1	already published at pub.dev

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[github-actions]

PR Health

Breaking changes ✔️

Details

Package	Change	Current Version	New Version	Needed Version	Looking good?
firehose	None	0.7.0	0.8.0	0.7.0	✔️

Changelog Entry ✔️

Details

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

Coverage ✔️

Details

File	Coverage
pkgs/firehose/lib/src/health/health.dart	💚 93 %

This check for test coverage is informational (issues shown here will not fail the PR).

License Headers ✔️

Details

// Copyright (c) 2024, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

Files
no missing headers

All source files should start with a license header.

Package publish validation ✔️

Details

Package	Version	Status
package:firehose	0.8.0	ready to publish
package:dart_flutter_team_lints	2.1.1	already published at pub.dev

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.