packaging: include git-x509-cert and replace fake windows msi

[droren]

This PR fixes the release packaging so shipped artifacts match the current functionality of the project.

Problem

Two release issues existed:

• git-x509-cert was implemented and documented, but was not included in release archives or Linux packages.
• the Windows “MSI” artifact was not a real installer at all; the workflow created a dummy placeholder file with an .msi extension.

What This PR Changes

• Updates .github/workflows/build-binaries.yml
  • macOS release tarball now contains:
    • smimesign
    • git-x509-cert
  • Linux release tarballs now contain:
    • smimesign
    • git-x509-cert
  • Windows release zip files now contain:
    • smimesign.exe
    • git-x509-cert.exe
  • removes the fake Windows MSI placeholder
  • builds a real Windows installer using Inno Setup
  • uploads the resulting installer as a .exe
• Updates windows-installer/inno-setup-smimesign-installer.iss
  • installer now includes both:
    • smimesign.exe
    • git-x509-cert.exe
• Updates nfpm.yaml
  • Linux RPM now installs:
    • /usr/bin/smimesign
    • /usr/bin/git-x509-cert

User-Facing Outcome

After this PR:

• users downloading release archives get both the signing tool and the commit-certificate inspection tool
• Linux package users get git-x509-cert installed alongside smimesign
• Windows users get a real installer instead of an empty fake .msi
• the installer installs both binaries into the application directory and PATH flow already handled by Inno Setup

Notable Behavior Change

• There is no longer any claim of a Windows MSI in the release workflow.
• The supported Windows installer artifact is now an Inno Setup .exe, which is the installer format actually implemented in this
  repository.