rust-toolchain: Bump toolchain version to 1.70.0

which, compared to the previous 1.69.0, as the following impacts on
`zellij`:

1. [Enable sparse registry checkout for crates.io by default][1]

This drastically increases the time to first build on a fresh rust
installation/a rust installation with a clean cargo registry cache.
Previously it took about 75s to populate the deps/cache (with `cargo
fetch --locked` and ~100 MBit/s network), whereas now the same process
takes ~10 s.

2. [The `OnceCell` type is now part of std][2]

In theory, this would allow us to cut a dependency from `zellij-utils`,
but the `once_cell` crate is pulled in by another 16 deps, so there's no
point in attempting it right now.

Build times and binary sizes are unaffected by this change compared to
the previous 1.69.0 toolchain.

[1]: rust-lang/cargo#11791
[2]: https://doc.rust-lang.org/stable/std/cell/struct.OnceCell.html

rust-toolchain.toml

@@ -1,5 +1,5 @@
 # This file is updated by `update-toolchain.sh`
 [toolchain]
-channel = "1.69.0"
+channel = "1.70.0"
 components = ["rustfmt", "clippy", "rust-analysis"]
 targets = ["wasm32-wasi", "x86_64-unknown-linux-musl"]