ci: tell gpg which key to sign with

[synackd]

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Checklist

• My code follows the style guidelines of this project
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• I have added tests that prove my fix is effective or my feature works
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Description

Attempt to fix "gpg: signing failed: No secret key" error by specifying which key to use for signing (namely, the one that was imported).

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

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