NEAR Contracts: Fix the integration tests

[MantisClone]

Problem

The integration tests fail on the main branch

status: Failure(Action #4: PrepareError: Error happened while deserializing the module.),

[MantisClone]

This error seems to indicate that we're using opcodes outside the wasm-core-1 specification

Source: near/nearcore#8358 (comment)

[MantisClone]

near/nearcore#8358 (comment)

I searched for ways to disable the sign-ext opcodes on the compiler and ran across this issue that seems to indicate that the option to disable sign-ext exists 👍 but that it's not working. 😢 rust-lang/rust#109807

Thus, I will likely pursue the wasm-opt --signext-lowering approach.

[MantisClone]

This issue was "bandaid" fixed by #16 which downgrades to Rust 1.67 - thus avoiding the later versions where sign-ext opcodes are added by default.

This issue should remain open until we've found a long-term fix.