Make BuildOptions immutable

[gregestren]

Configuration transitions, which are a core building block of same-build multiplatform builds, work by transforming an input set of BuildOptions into an appropriate output set.

It's important that this transformation doesn't mutate the input set, since that would inadvertently apply the changes to the source configuration, which messes the whole transition up (in a best case) and destroys Bazel's internals (in a worst).

Unfortunately, the BuildOptions class isn't immutable. And it isn't easy to make it immutable with the way it's currently used.

Starlark build configuration should eventually refactor this severely enough to make it naturally immutable. This issue tracks both that sub-effect (as a subset of #5574) and interim efforts to "harden up" BuildOptions today to at least make it "inconvenient" to mutate if not totally impossible.

[github-actions]

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[github-actions]

This issue has been automatically closed due to inactivity. If you're still interested in pursuing this, please reach out to the triage team (@bazelbuild/triage). Thanks!