Use variable names directly in the format strings (#7906)

# Description

Lint: `clippy::uninlined_format_args`

More readable in most situations.
(May be slightly confusing for modifier format strings
https://doc.rust-lang.org/std/fmt/index.html#formatting-parameters)

Alternative to #7865

# User-Facing Changes

None intended

# Tests + Formatting

(Ran `cargo +stable clippy --fix --workspace -- -A clippy::all -D
clippy::uninlined_format_args` to achieve this. Depends on Rust `1.67`)

crates/nu-command/src/strings/encode_decode/base64.rs

@@ -93,8 +93,7 @@ fn action(
         not_valid => return Value::Error { error:ShellError::GenericError(
             "value is not an accepted character set".to_string(),
             format!(
-                "{} is not a valid character-set.\nPlease use `help hash base64` to see a list of valid character sets.",
-                not_valid
+                "{not_valid} is not a valid character-set.\nPlease use `help hash base64` to see a list of valid character sets."
             ),
             Some(config_character_set.span),
             None,

crates/nu-command/src/strings/encode_decode/encoding.rs

@@ -62,8 +62,7 @@ fn parse_encoding(span: Span, label: &str) -> Result<&'static Encoding, ShellErr
     match Encoding::for_label_no_replacement(label.as_bytes()) {
         None => Err(ShellError::GenericError(
             format!(
-                r#"{} is not a valid encoding"#,
-                label
+                r#"{label} is not a valid encoding"#
             ),
             "invalid encoding".into(),
             Some(span),