Optimize MIRI execution time in CI

[alexcrichton]

• Disable Cranelift optimizations and use single_pass register allocation by default.

• Ignore a number of tests that are compiling wasm which we generally don't want to do under MIRI.

[github-actions]

Label Messager: wasmtime:config

It looks like you are changing Wasmtime's configuration options. Make sure to
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