Sign the Windows slicec binary in release builds

[pepone]

The release workflow uploads slicec.exe to download.zeroc.com without an Authenticode signature, and IceRpc.Slice.Tools bundles it unchanged. The tools/windows-x64/slicec.exe in IceRpc.Slice.Tools 0.6.0 on nuget.org is NotSigned, while the managed assemblies in the same package carry ZeroC's signature. This was found while fixing icerpc/icerpc-csharp#4949.

This PR signs the Windows binary on the Windows build leg, before the artifact upload, with the same Trusted Signing account, certificate profile and action version as icerpc-csharp's build-packages.yml:

• Releases (any quality other than nightly): signing is mandatory.
• Nightly builds: signing is optional, through a new authenticode_sign input (default false) on workflow_call and workflow_dispatch. The scheduled nightly doesn't set it, so it stays unsigned. A manual nightly run can opt in.
• Verify step: after signing, a step fails the build unless Get-AuthenticodeSignature reports Valid, so a signing problem can't lead to an unsigned binary being uploaded. The upload job needs build, so nothing is published in that case.

The signing action is pinned to a commit SHA, as in icerpc-csharp, because it receives the signing credentials.

Before merging

The AZURE_TENANT_ID, AZURE_CLIENT_ID and AZURE_CLIENT_SECRET secrets are organization secrets that are not currently shared with this repository. An org admin needs to add icerpc/slicec to their selected repositories. Until then, a non-nightly release fails at the signing step. The scheduled nightly doesn't sign, so it isn't affected.

Testing

• actionlint 1.7.12 reports no issues.
• The signing policy script gives true for stable and RC with any input value, false for nightly by default, and true for nightly with authenticode_sign: true.
• The verify step's PowerShell passes on a ZeroC-signed assembly from nuget.org and fails on the unsigned 0.4.0 slicec.exe.
• The signing step itself can only run here once the secrets are shared. A manual workflow_dispatch with quality: nightly and authenticode_sign: true exercises it without publishing a stable release.

The workflow is the same on 0.4.x, so this commit cherry-picks cleanly and a 0.4.1 release would ship a signed slicec.exe.

What's Changed

None - release pipeline change: the published Windows slicec binary is now Authenticode-signed; the compiler itself is unchanged.

[Copilot]

🔵 Needs a closer look

Release signing and publication gating depend on organization secrets and warrant final human review.

Pull request overview

Updates the release workflow to Authenticode-sign Windows slicec.exe before publication.

Changes:

• Adds configurable signing for releases and nightly builds.
• Verifies signatures before artifact upload.
• Blocks publication when required signing fails.

File summaries

File	Summary
.github/workflows/build-release.yml	Adds signing policy, Azure Trusted Signing, verification, and release gating.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Lite

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