x/build/cmd/relui: add best-effort updating of tagx:ignore'd dependencies

[dmitshur]

The monthly tagging workflow skips updating modules annotated with tagx:ignore. This was done out of necessity to break cycles in the module graph (CL 439136).

This means those golang.org/x modules are left to be upgraded manually and may end up on fairly old versions.

It should be quite viable to add best-effort updating of these modules to at least their latest version available at the start of a given tagging workflow.

CC @golang/release.

[gabyhelp]

Related Issues

• x/build/cmd/relui: dynamically respond to nested modules in monthly golang.org/x tagging workflow #68873 (closed)
• x/build: update x/ dependencies in untagged x/ repos when applying tags #56530 (closed)
• x/build/cmd/relui: tag golang.org/x repos continuously #48523 (closed)

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