Roll back once_cell to 1.19.0.

Roll back `wgpu`'s dependencies on `once_cell` from 1.20.1 to 1.19.0.

Version 1.20.1 of `once_cell` added a more complex conditional
dependency on `portable-atomic`, which causes `cargo metadata` to
incorrectly list `portable-atomic` as a dependency even though the
given `once_cell` features are not enabled.

The Firefox source tree uses `cargo vet` to enforce supply-chain
auditing. Since `cargo vet` depends on `cargo metadata` to tell it
what crates are going to be included in the tree, the extraneous
dependency above adds `portable-atomic` to the set of sources we must
audit. Since `portable-atomic` is roughly 50kloc, we would like to
avoid this.

Nothing in `wgpu` actually needs `once_cell` 1.20; it was upgraded by
Dependabot. So the simplest workaround for the moment is to roll back
the version.

Cargo.toml

@@ -104,7 +104,7 @@ nanorand = { version = "0.7", default-features = false, features = ["wyrand"] }
 noise = { version = "0.8", git = "https://github.com/Razaekel/noise-rs.git", rev = "c6942d4fb70af26db4441edcf41f90fa115333f2" }
 nv-flip = "0.1"
 obj = "0.10"
-once_cell = "1.20.1"
+once_cell = "1.19.0"
 parking_lot = "0.12.1"
 pico-args = { version = "0.5.0", features = [
     "eq-separator",