ci: skip publishing if behind main

[tido64]

Description

Nx Release does not check whether it's behind main before performing Git operations.

Platforms affected

• Android
• iOS
• macOS
• visionOS
• Windows

Test plan

Run the following command in a Bash shell:

if git merge-base --is-ancestor origin/trunk HEAD; then echo PUBLISH; fi

PUBLISH should only be printed if we're in sync with origin/trunk.