rustbuild: add config.toml option to disable codegen tests #36234
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Fixes #36232.
I think it worked? Here's a build log where I tried to bootstrap, it crashed, then I added the setting to config.toml and it continued: https://gist.github.com/durka/cbf97cf04b8e065f1a2cfda4c1b6bf95
r? @alexcrichton