Use newer rust toolchain channel to allow compiling stwo-example blank project

[gabrielbosio]

By following the current Hello World page, once I have the new Cargo project with the rust-toolchain.toml and Cargo.toml files set as described there, when running:

cargo build

or:

cargo build --release

both commands fail with compilation errors:

   Compiling stwo v0.1.1 (https://github.com/starkware-libs/stwo.git?rev=75a6b0ac9bcc7101d8658445dded51923ab2586f#75a6b0ac)
error[E0599]: no method named `get_disjoint_mut` found for array `[BaseColumn; 4]` in the current scope
   --> /Users/gabriel/.cargo/git/checkouts/stwo-59e22971a65c0edb/75a6b0a/crates/stwo/src/prover/backend/simd/column.rs:482:14
    |
480 |           let [a, b, c, d] = self
    |  ____________________________-
481 | |             .columns
482 | |             .get_disjoint_mut([0, 1, 2, 3])
    | |             -^^^^^^^^^^^^^^^^ method not found in `[BaseColumn; 4]`
    | |_____________|
    |

error[E0015]: cannot call non-const associated function `Simd::<u32, 16>::splat` in constants
  --> /Users/gabriel/.cargo/git/checkouts/stwo-59e22971a65c0edb/75a6b0a/crates/stwo/src/prover/backend/simd/blake2s.rs:36:5
   |
36 |     u32x16::splat(LEAF_INITIAL_STATE[0]),
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: calls in constants are limited to constant functions, tuple structs and tuple variants

[... more compilation errors ...]

Some errors have detailed explanations: E0015, E0599, E0658.
For more information about an error, try `rustc --explain E0015`.
error: could not compile `stwo` (lib) due to 37 previous errors

Changing the Rust toolchain channel used with the one set in the finished example fixes the compilation errors mentioned.

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This change is