chore: use new parallel build feature of jsii

[rix0rrr]

Improve build times by using the new "build-all-at-once" feature of
jsii-pacmak.

Goes together with aws/jsii#849.

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[rix0rrr]

(Obviously this will not compile with the current jsii version)

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[rix0rrr]

At the moment this PR brings the build time down from ~45mins to ~30mins. We should probably be parallelizing the checking of examples.