doc: Note expired system certificate issue in how-to/code-signing/macOS

[hyuri]

When an intermediate certificate is expired in the system — seems to be the Developer ID certificate —, instead of This certificate is valid, one will see "Developer ID Application: [...]" certificate is not trusted instead.

This adds a note on how to solve that issue — upgrading the system, or installing a fresh version of the intermediate certificate.

PR Checklist:

• I will abide by the BeeWare Code of Conduct
• I have read and have followed the CONTRIBUTING.md file
• This PR was generated or assisted using an AI tool

Assisted-by:

[freakboy3742]

Thanks for the PR! It's currently failing CI because of the need for a changenote, and a spelling issue (adding PKI to the spelling dictionary).

The only other thought (and I'm guessing you've fixed the problem, so you don't have the ability to test this any more) - can we catch this failure mode at runtime and provide advice? Do you still have logs from these failed runs?

[hyuri]

[...] I'm guessing you've fixed the problem, so you don't have the ability to test this any more) [...]

Yes, problem fixed on my end so I can't test this any further.

The only other thought [...] can we catch this failure mode at runtime and provide advice? Do you still have logs from these failed runs?

This part of the process is done without running briefcase, so there are no failed runs.
The issue only manifests itself inside Keychain Access, after you install your Developer ID Application certificate.

[hyuri]

[...] and a spelling issue (adding PKI to the spelling dictionary).

I thought you meant you were adding it. Sorry. Done now.

[freakboy3742]

I've pushed a couple of minor tweaks, but this is great! Thanks!