bump rust-toolchain to 1.75.0 (#12258)

# Description

With the release of Rust 1.77.0 today we're able to bump the
rust-toolchain for nushell to 1.75.0.

# User-Facing Changes
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# Tests + Formatting
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> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```
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crates/nu-protocol/src/errors/labeled_error.rs

@@ -191,7 +191,7 @@ impl fmt::Display for LabeledError {
 
 impl std::error::Error for LabeledError {
     fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
-        self.inner.get(0).map(|r| r as _)
+        self.inner.first().map(|r| r as _)
     }
 }
 

rust-toolchain.toml

@@ -16,4 +16,4 @@ profile = "default"
 # use in nushell, we may opt to use the bleeding edge stable version of rust.
 # I believe rust is on a 6 week release cycle and nushell is on a 4 week release cycle.
 # So, every two nushell releases, this version number should be bumped by one.
-channel = "1.74.1"
+channel = "1.75.0"