lint: shellcheck files with shell shebang #36742

The current shellcheck lint silently ignores all files with a shell shebang that are not ending in .sh.

Fix it by linting them, which also simplifies the script, because manually appending of guix scripts can be dropped.

Also, remove global shellcheck exclude of SC2162 (read -r).

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36742
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36742.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36742#author-ralph

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #35686 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/35686.svg">} (lint: have git-subtree-check check for backportability by Sjors)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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Concept ACK.