Allow 'latest' as wasm-opt version

[Madoshakalaka]

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I did some manual testing on my system with and without system-installed wasm-opt; with and without using the latest keyword, with specified versions etc. All seem to work fine.

~~also bumped default wasm-opt version to version_123~~

~~I think also #904 can be closed since from my testing new wasm_opt versions processe the binary successfully without any workarounds.~~

~~PS. tried both version_122 and version_123. And 123 is soooo fast. Did a rough test and it's like 30 seconds vs 5 seconds, as advertised~~

~~> Given the big speedup in our official release binaries for Linux, this seems useful to get to users quickly.~~

~~I think it's a worthwhile bump.~~

[ctron]

I think it would be good splitting the default version update and the addition of "latest" into different PRs. Updating the default version seems like a quick one. While the other requires more thinking.

[Madoshakalaka]

anything lacking? I'm not familiar with the testing code here but I can explore if that's necessary.