fix(core): allow overriding NX_PARALLEL with --parallel

[AgentEnder]

Current Behavior

NX_PARALLEL overrides --parallel flag

Expected Behavior

--parallel is more specific so it should override NX_PARALLEL

Related Issue(s)

Fixes #26903

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✅ Successfully ran 4 targets

• nx affected --targets=lint,test,build,e2e,e2e-ci --base=99a488ae75ccac77956e7ec1f3fd1e8f5e2f6317 --head=6ab3bb80e0292d29869fbc2f629135cd6fd382e5 --parallel=3
• nx run-many -t check-imports check-commit check-lock-files check-codeowners documentation --parallel=1 --no-dte
• nx-cloud record -- nx format:check --base=99a488ae75ccac77956e7ec1f3fd1e8f5e2f6317 --head=6ab3bb80e0292d29869fbc2f629135cd6fd382e5
• nx documentation --no-dte

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[SebastienGllmt]

Not fully convinced of this change. The rationale for NX_PARALLEL's creation was so that you could set a high parallel for dev machines, while setting a low NX_PARALLEL for CI so that it doesn't blow up on cheap CI machines

This PR means that you effectively have to always do --parallel=${NX_PARALLEL:-3} on all your calls now to maintain the high parallel on desktop with low parallel on CI, but if you now need to manually specify this everywhere it's not as clear why NX_PARALLEL is a built-in feature

Maybe there is another use-case that I'm not thinking of

[github-actions]

This pull request has already been merged/closed. If you experience issues related to these changes, please open a new issue referencing this pull request.