fix: output valid turbo.json for prune with Boundaries definition #6479

Job	Run time
Find path changes	16s
Rust lints	8m 19s
Turborepo Integration (macos-13)	25m 40s
Turborepo Integration (ubuntu-latest)	10m 28s
Turborepo Rust testing on windows	15m 27s
@turbo/types codegen check	39s
Turborepo rust check	4m 58s
Turborepo Rust testing on ubuntu	5m 42s
Turborepo Rust testing on macos	11m 58s
Turborepo Integration (windows-latest)	25m 12s
`with-tailwind` example	0s
`non-monorepo` example	0s
`basic` example	0s
`kitchen-sink` example	0s
`with-svelte` example	0s
Turborepo Test Summary	4s
Cleanup / cleanup	7s
	1h 48m 50s

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