Fix lint failures with Rust 1.76

Fixes the following failure on main:

 error: writing `&Vec` instead of `&[_]` involves a new object where a slice will do
   --> buildpacks/php/src/platform/generator.rs:177:28
    |
177 |     platform_repositories: &Vec<Url>,
    |                            ^^^^^^^^^ help: change this to: `&[Url]`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#ptr_arg
    = note: `-D clippy::ptr-arg` implied by `-D warnings`

As seen in: https://github.com/heroku/buildpacks-php/actions/runs/8109044448/job/22163445587?pr=82#step:5:275

Docs: https://rust-lang.github.io/rust-clippy/master/index.html#/ptr_arg

GUS-W-15212817.

heroku / buildpacks-php

Fix lint failures with Rust 1.76 #83