fix: platform constraint for ECDH #224

Job	Run time
Analyze (cryptography, beta)	32s
Analyze (cryptography, 3.3.0)	44s
Analyze (cryptography_test, 3.3.0)	39s
Analyze (cryptography_test, beta)	30s
Analyze (jwk, 3.3.0)	36s
Analyze (cryptography_flutter_integration_test, 3.35.0)	1m 4s
Analyze (jwk, beta)	28s
Analyze (cryptography_flutter/example, beta)	42s
Analyze (cryptography_flutter, beta)	46s
Analyze (cryptography_flutter_integration_test, beta)	44s
Analyze (cryptography_flutter, 3.35.0)	52s
Analyze (cryptography_flutter/example, 3.35.0)	49s
PANA (cryptography)	1m 55s
PANA (jwk)	1m 52s
Test (cryptography, dart2wasm)	4m 2s
Test (jwk, dart2wasm)	32s
Test (cryptography, vm)	48s
Test (jwk, dart2js)	30s
Test (cryptography, dart2js)	2m 43s
Test (jwk, vm)	19s
PANA (cryptography_flutter)	1m 41s
	22m 48s

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