next-upgrade: do not add `--turbopack` flag when `--turbo` exists in … #15211

Job	Run time
deploy-target	11s
build-wasm (web)	24s
build	2m 11s
build-wasm (nodejs)	22s
stable - x86_64-unknown-linux-gnu - node@16	45s
Upload Turbopack Bytesize metrics to Datadog	0s
Deploy preview tarball	2m 2s
Potentially publish release	0s
Deploy examples	0s
Release Stats	0s
	5m 55s

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