ci: fix signing on Windows in the release workflow

[dsanders11]

This PR:

• Switches the bespoke setup of DigiCert's signing tools to using the digicert/ssm-code-signing code action
• Properly exports the SM_CLIENT_CERT_FILE environment variable and makes use of it
• Liberally sets environment variables for the Windows steps
• Fixes the Linux build step having a wrong if field
• Reverts build: use @electron/windows-sign #1785, I still couldn't get that working 🤷

Lessons learned:

• In GHA, powershell and pwsh are different versions of PowerShell, not just aliases for each other
• If some of the environment variables aren't set, things fail with entirely unhelpful error messages

[coveralls]

coverage: 80.223% (+0.01%) from 80.211%
when pulling d67e695 on ci/fix-windows-signing-release
into c23b792 on main.