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/conversions/into/datetime.rs

@@ -232,7 +232,7 @@ fn action(input: &Value, args: &Arguments, head: Span) -> Value {
             return Value::Error {
                 error: ShellError::UnsupportedInput(
                     "timestamp is out of range; it should between -8e+12 and 8e+12".to_string(),
-                    format!("timestamp is {:?}", ts),
+                    format!("timestamp is {ts:?}"),
                     head,
                     // Again, can safely unwrap this from here on
                     input.expect_span(),

crates/nu-command/src/conversions/into/int.rs

@@ -367,8 +367,7 @@ fn int_from_string(a_string: &str, span: Span) -> Result<i64, ShellError> {
                     "string".to_string(),
                     span,
                     Some(format!(
-                        r#"string "{}" does not represent a valid integer"#,
-                        trimmed
+                        r#"string "{trimmed}" does not represent a valid integer"#
                     )),
                 )),
             },

crates/nu-command/src/conversions/into/string.rs

@@ -206,7 +206,7 @@ fn action(input: &Value, args: &Arguments, span: Span) -> Value {
             if decimals {
                 let decimal_value = digits.unwrap_or(2) as usize;
                 Value::String {
-                    val: format!("{:.*}", decimal_value, val),
+                    val: format!("{val:.decimal_value$}"),
                     span,
                 }
             } else {