Turn off incremental Rust compilation

[emilk]

Incremental compilation blows up the size of the target folder,
but it is also buggy, causing spurious "link failures" that requires a complete clean of the target folder.

Note that the incremental part is for each individual crate. Even with CARGO_INCREMENTAL we only recompile the crates that we need to recompile (but we will recompile them in their entirety).

https://doc.rust-lang.org/cargo/reference/profiles.html#incremental

[github-actions]

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| Result | Commit | Link | Manifest |
| ------ | ------- | ----- |
| ❌ | | https://rerun.io/viewer/pr/12207 | +nightly +main |

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[teh-cmc]

FYI: There's a less hacky way than an environment variable that I use in my ~/.cargo:

[build]
rustc-wrapper = "/home/cmc/.cargo/bin/sccache"
incremental = false

[target.x86_64-unknown-linux-gnu]
linker = "clang"
rustflags = [
  "-Clink-arg=-fuse-ld=/usr/bin/mold",
  "-Csplit-debuginfo=unpacked",
  "-Ctarget-cpu=native",
  "-Cforce-unwind-tables=yes",
  "--cfg", "tokio_unstable",
]