fix(nx-ignore): check that yarn/pnpm are installed before using on Netlify

[jaysoo]

Netlify builds seem to have broken with recent changes. yarn and pnpm aren't available during the ignore build step.

This PR adds additional checks that yarn and pnpm are available before using them.

[nx-cloud]

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• nx affected --target=e2e --parallel --max-parallel=1 --exclude=nx-ignore-e2e,gatsby-e2e
• nx affected --target=test --parallel --max-parallel=2
• nx affected --target=build --parallel --max-parallel=3

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• nx affected --target=lint --parallel --max-parallel=3

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