systemd: build: Add sethostname workaround for AmazonLinux2

[cosmo0920]

In AmazonLinux2, it is built on top of coreos technology. So, this race condition bug is also existing there.
In this PR, I added a workaround which is proposed in #9506 (comment).

This workaround is only effective for AmazonLinux2.

Closes #9506

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