specgen: apply env_merge from containers.conf

[jiwahn]

Read env_merge values from containers.conf and apply them before processing CLI --env-merge flags.

Fixes: #28410
Depends on: containers/container-libs#791

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Does this PR introduce a user-facing change?

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[jiwahn]

This pr is depens on containers/container-libs#791 and I am waiting for it to be merged