fix: respect signoff in manifest config

[mysteq]

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Fixes #2333 and #2280 🦕

Changes the implementation of #2246 since signoff is only applicapbe to root in manifest config.

Output of debug-config after PR

...
separatePullRequests: true,
fork: false,
signoffUser: 'foo <foo@users.noreply.github.com>',
releaseLabels: [ 'autorelease: tagged' ],
labels: [ 'autorelease: pending' ],
...

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