lang/rust-nightly: Try to reduce memory usage/pressure

Try to reduce memory usage/pressure by only using one code generation
unit.

"This flag [codegen-units] controls how many code generation units
the crate is split into. It takes an integer greater than 0.

When a crate is split into multiple codegen units, LLVM is able to
process them in parallel. Increasing parallelism may speed up
compile times, but may also produce slower code. Setting this to
1 may improve the performance of generated code, but may be slower
to compile."

https://doc.rust-lang.org/rustc/codegen-options/index.html#codegen-units

PR: 256099
Suggested by: Daniel Engberg