fix: add missing explicit lifetime for rust 1.83 build

[chenrui333]

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---

👋 This PR is fixing the regression build issue against rust 1.83.0:

  error: elided lifetime has a name
     --> flux-core/src/ast/walk/mod.rs:183:47
      |
  181 | impl<'a> Node<'a> {
      |      -- lifetime `'a` declared here
  182 |     #[allow(missing_docs)]
  183 |     pub fn from_expr(expr: &'a Expression) -> Node {
      |                                               ^^^^ this elided lifetime gets resolved as `'a`
      |
  note: the lint level is defined here
     --> flux-core/src/lib.rs:1:38
      |
  1   | #![cfg_attr(feature = "strict", deny(warnings, missing_docs))]
      |                                      ^^^^^^^^
      = note: `#[deny(elided_named_lifetimes)]` implied by `#[deny(warnings)]`
  
  error: elided lifetime has a name
    --> flux-core/src/parser/mod.rs:44:37
     |
  42 | impl<'input> Parser<'input> {
     |      ------ lifetime `'input` declared here
  43 |     /// Instantiates a new parser with the given string as input.
  44 |     pub fn new(src: &'input str) -> Parser {
     |                                     ^^^^^^ this elided lifetime gets resolved as `'input`

• relates to rust 1.83.0 Homebrew/homebrew-core#199379
• fixes 0.195.2 failed to build against rust 1.83.0 #5515