Pin rust-version in CI and local builds to the version used to build Firefox

[msirringhaus]

Unlike #314, this will also use the 'right' toolchain locally.

[jschanck]

Thanks! We can only use features present in the "MSRV" column from https://firefox-source-docs.mozilla.org/writing-rust-code/update-policy.html. So it should be 1.66, not 1.73.