Update rust-toolchain to nightly of 2023-06-26

This updates `libtock-rs` to a relatively old Rust toolchain, which
happens to ship a rustc v1.64.0-nightly such that the latest `elf2tab`
release can build with it (v0.11.0). However, it's not too recent for
Miri to complain about unsupported integer-to-pointer casts in
strict-provenance mode:

    running 5 tests
    test tests::driver_check ... error: unsupported operation: integer-to-pointer casts and `ptr::from_exposed_addr` are not supported with `-Zmiri-strict-provenance`
       --> /home/leons/dev/libtock-rs/platform/src/register.rs:25:18
        |
    25  |         Register(value as *mut ())
        |                  ^^^^^^^^^^^^^^^^ integer-to-pointer casts and `ptr::from_exposed_addr` are not supported with `-Zmiri-strict-provenance`
        |
        = help: use Strict Provenance APIs (https://doc.rust-lang.org/nightly/std/ptr/index.html#strict-provenance, https://crates.io/crates/sptr) instead
        = note: backtrace:
        = note: inside `<libtock_platform::Register as core::convert::From<u32>>::from` at /home/leons/dev/libtock-rs/platform/src/register.rs:25:18
        = note: inside `<u32 as core::convert::Into<libtock_platform::Register>>::into` at /home/leons/.rustup/toolchains/nightly-2022-08-02-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/convert/mod.rs:550:9

This serves as a bridge-update to get CI running again, until a proper
fix for the above Miri error is integrated (see [1]).

[1]: tock#481

rust-toolchain

@@ -1,7 +1,7 @@
 [toolchain]
 # See https://rust-lang.github.io/rustup-components-history/ for a list of
 # recently nightlies and what components are available for them.
-channel = "nightly-2022-06-10"
+channel = "nightly-2022-06-26"
 components = ["clippy", "miri", "rustfmt"]
 targets = ["thumbv7em-none-eabi",
            "riscv32imac-unknown-none-elf",