[Spawner] Block further SIGINTs with unload_on_kill option (backport #3075)

[mergify]

Fixes: #2063

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This is an automatic backport of pull request #3075 done by Mergify.

[mergify]

Cherry-pick of 2794132 has failed:

On branch mergify/bp/jazzy/pr-3075
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	both modified:   controller_manager/controller_manager/spawner.py

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