QNAP now requires builds to be signed with an HSM.
This removes support for signing with a local keypair.
This adds support for signing with a Google Cloud hosted key.
The key should be an RSA key with protection level `HSM` and that uses PSS padding and a SHA256 digest.
The GCloud project, keyring and key name are passed in as command-line arguments.
The GCloud credentials and the PEM signing certificate are passed in as Base64-encoded command-line arguments.
Updates tailscale/corp#23528
Signed-off-by: Percy Wegmann <percy@tailscale.com>
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