fix(core): do not run postinstall unless it is the main nx package

[FrozenPandaz]

Current Behavior

The postinstall script runs for all instances of Nx in a workspace.

Expected Behavior

The postinstall script only runs for the main instance of nx in a workspace, not the instances of nx that are a dependency of other packages.

Related Issue(s)

Fixes #

[nx-cloud-staging]

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✅ Successfully ran 12 targets

• nx affected --target=e2e --base=23de788b5e8c69adbbc8a860fefc99d481ef6155 --head=f1c2b21c675037f223fbb9a75c29e2e1e4aa4b55 --exclude=e2e-storybook,e2e-storybook-angular,e2e-react-native,e2e-detox,e2e-make-angular-cli-faster --parallel=1
• nx affected --target=test --base=23de788b5e8c69adbbc8a860fefc99d481ef6155 --head=f1c2b21c675037f223fbb9a75c29e2e1e4aa4b55 --parallel=1
• nx affected --target=build --base=23de788b5e8c69adbbc8a860fefc99d481ef6155 --head=f1c2b21c675037f223fbb9a75c29e2e1e4aa4b55 --parallel=3
• nx affected --target=lint --base=23de788b5e8c69adbbc8a860fefc99d481ef6155 --head=f1c2b21c675037f223fbb9a75c29e2e1e4aa4b55 --parallel=3
• nx-cloud record -- yarn depcheck
• nx-cloud record -- yarn nx workspace-lint
• nx-cloud record -- yarn check-lock-files
• nx-cloud record -- yarn check-commit
• nx-cloud record -- yarn nx format:check --base=23de788b5e8c69adbbc8a860fefc99d481ef6155 --head=f1c2b21c675037f223fbb9a75c29e2e1e4aa4b55
• nx-cloud record -- yarn check-imports
• nx-cloud record -- yarn documentation
• nx build typedoc-theme

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Name	Status	Preview	Updated
nx-dev	✅ Ready (Inspect)	Visit Preview	Oct 13, 2022 at 11:23PM (UTC)

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