Fix the missing `parallel` feature flag for ark-serialize #2864

Job	Run time
Check Documentation	16s
Check (stable)	8s
Check (nightly)	25s
Check Style	5s
List directories for parallelizing tests	2s
Test (stable)	19s
Test (nightly)	25s
Test assembly	21s
Test (cp6_782/)	2m 18s
Test (mnt6_753/)	2m 20s
Test (mnt4_753/)	2m 22s
Check no_std	33s
Test (./bw6_761)	2m 13s
Test (./ed_on_mnt4_298)	1m 13s
Test (./mnt6_298)	41s
Test (./mnt4_298)	45s
Test (./secp256r1)	1s
Test (./pallas)	36s
Test (./secp384r1)	1s
Test (./vesta)	1s
Test (./ed_on_bn254)	1m 9s
Test (./ed25519)	45s
Test (./ed_on_mnt4_753)	49s
Test (./ed_on_bls12_381)	1m 13s
Test (./ed_on_bw6_761)	50s
Test (./bw6_767)	2m 9s
Test (./ed_on_cp6_782)	1m 30s
Test (./curve25519)	1m 8s
Test (./grumpkin)	48s
Test (./secp256k1)	16s
Test (./secq256k1)	1s
Test (./ed_on_bls12_377)	1m 3s
Test (./ed_on_bls12_381_bandersnatch)	1m 4s
Test (./bn254)	2m 9s
Test (./bls12_377)	2m 9s
Test (./bls12_381)	2m 9s
	34m 17s

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