[CI] [Debian] use -j1 on js-of-ocaml (#1947) #654

Job	Run time
docker-master	1h 25m 37s
publish-standalone-dry-run	15s
test-standalone (master, dev, default)	1m 31s
build-js-of-ocaml (master, 4.11.1)	13m 30s
generated-files (master)	1m 56s
build-wasm-of-ocaml (master, 4.14.1)	2h 16m 36s
test-amd64	2h 17m 2s
standalone-haskell (master)	46m 51s
validate-docker-master	24m 40s
deploy-js-wasm-of-ocaml	29s
docker-check-all	8s
	7h 28m 35s

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