Fix BuildChainCustomTrustStore test

[vcsjones]

The test BuildChainCustomTrustStore started failing in Alpine 3.22.

This is failing because the root certificate of the chain, "C=IE, O=Baltimore, OU=CyberTrust, CN=Baltimore CyberTrust Root" expired May 12 23:59:00 2025. Since the root certificate is expired, it was completely removed from the trust store in an update to Alpine.

This unit test was relying on the certificate being in the system trust store, which is no longer true.

This pull request changes the "use the system trust store" case to permit an untrusted root. This is consistent with what we are doing in other tests that use this chain:

runtime/src/libraries/System.Security.Cryptography/tests/X509Certificates/ChainTests.cs

Lines 46 to 49 in a436284

	chain.ChainPolicy.ExtraStore.Add(unrelated);
	chain.ChainPolicy.ExtraStore.Add(microsoftDotComRoot);
	chain.ChainPolicy.ExtraStore.Add(microsoftDotComIssuer);
	chain.ChainPolicy.VerificationFlags = X509VerificationFlags.AllowUnknownCertificateAuthority;

Contributes to #117723

[Copilot]

Pull Request Overview

This PR fixes the BuildChainCustomTrustStore test that started failing in Alpine 3.22 due to the expiration of the Baltimore CyberTrust Root certificate on May 12, 2025. The certificate was subsequently removed from the system trust store, causing the test to fail when it expected the certificate to be available.

• Modifies the system trust store test case to allow untrusted root certificates
• Adds appropriate verification flags and chain status handling for expired certificates
• Aligns the test behavior with other similar tests in the codebase

[dotnet-policy-service]

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[vcsjones]

Passed on Alpine 3.22.

We should back port this.

[vcsjones]

/backport to release/9.0-staging

[vcsjones]

/backport to release/8.0-staging

[github-actions]

Started backporting to release/8.0-staging: https://github.com/dotnet/runtime/actions/runs/16347511153

[github-actions]

Started backporting to release/9.0-staging: https://github.com/dotnet/runtime/actions/runs/16347508992