Fix CI and cargo build errors

[wenyuzhao]

• Fix GitHub actions so it runs on ubuntu-20.04 and can pass all the v8 unit tests (with mmtk nogc). But it still pins a relatively old v8 commit.
• Fix all the cargo build errors
• Update rust-toolchain to match mmtk-core
• Update mmtk-v8 version to v0.18.0

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Note that v8 with mmtk-v8 still can't build on ubuntu-22.04, until we move forward the pinned v8 commit to a version that supports 22.04.

[qinsoon]

Thanks for the PR!