Remove use of shared mutable static

[jeffcharles]

Description of the change

Changes the codegen module to use an immutable static OnceLock for a MemoryInputPipe instead of a mutable static OnceLock on the WasiCtx. This shouldn't have any functional impact on the CLI. Integration test time increased from 51.04 seconds to 51.94 seconds which seems like a minor increase considering the number of tests involved.

Why am I making this change?

Using shared mutable statics in Rust is very discouraged. We had to disable a lint rule to allow this use. We also can remove some unsafe blocks too.

Checklist

• I've updated the relevant CHANGELOG files if necessary. Changes to javy-cli and javy-plugin do not require updating CHANGELOG files.
• I've updated the relevant crate versions if necessary. Versioning policy for library crates
• I've updated documentation including crate documentation if necessary.