Fix pipe transmission

[DL6ER]

What does this implement/fix?

Fix pipe transmission from forked dnsmasq process validation changed configurations. This had no real consequences but could lead to missing characters in error messages describing how a new config is invalid (if a user added something invalid). However, as we are testing for this, the CI sometimes automatically retried the jobs most often resulting in a success, eventually.

Example for such an issue: (mind the missing initial 'N' of Name does not...)

This PR ensures we properly flush the pipe (by sending 32 bytes of Domestos down the line) before using it so no dirt gets stuck on its way.

Note: Interestingly enough, this has never happened on any of the native builds (not even on the ARM builds on the new ARM64 self-hosted runners) but always only in the emulated buildx environments. Currently, this only affects riscv64.

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Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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5. I have squashed any insignificant commits. (git rebase)

Checklist:

• [x] The code change is tested and works locally.
• [x] I based my code and PRs against the repositories developmental branch.
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[github-actions[bot]]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[DL6ER]

Closing until we're ready to look this again