Introduce rust-toolchain.toml

[mversic]

          How about we commit a `rust-toolchain.toml` file so that the developers get the rust version corresponding to what we have in CI right away?

[toolchain]
channel = "nightly-2024-09-09"

Originally posted by @DCNick3 in #5047 (comment)

[mversic]

Why did we remove it? @s8sato do you remember?

[s8sato]

We had a docker build problem with the toolchain file, but as far as I remember we didn't figure out the cause. I guess it was maybe a toolchain mismatch between environments. I think it's worth introducing again, if we can eliminate hardcoded toolchains except for the toml content. Otherwise, we should be careful to keep hardcoded toolchains consistent with the toml content