fix: platform constraint for ECDH #223

Job	Run time
Analyze (cryptography_flutter, beta)	1m 39s
Analyze (cryptography_flutter, 3.35.0)	51s
Analyze (cryptography_flutter/example, beta)	47s
Analyze (cryptography_test, 3.3.0)	42s
Analyze (jwk, 3.3.0)	1m 8s
Analyze (cryptography, beta)	33s
Analyze (cryptography_flutter_integration_test, beta)	49s
Analyze (cryptography_test, beta)	29s
Analyze (cryptography, 3.3.0)	42s
Analyze (cryptography_flutter_integration_test, 3.35.0)	53s
Analyze (cryptography_flutter/example, 3.35.0)	52s
Analyze (jwk, beta)	28s
Test (cryptography, dart2js)	2m 44s
PANA (cryptography)	2m 1s
PANA (jwk)	1m 46s
Test (cryptography, vm)	1m 32s
Test (cryptography, dart2wasm)	2m 33s
Test (jwk, vm)	19s
Test (jwk, dart2wasm)	35s
Test (jwk, dart2js)	35s
PANA	-1s
	21m 57s

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