tls12-download.exe: Enforce that the signing cert uses SHA-2. #1970

Job	Run time
format	3m 2s
json-schema	1m 6s
builds / build (windows-2022, windows-ci)	19m 45s
builds / build (ubuntu-20.04, linux-ci)	9m 26s
builds / build (macos-13, macos-ci)	13m 20s
	46m 39s

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