lint workflow failing on PR without code changes

[technicalpickles]

ie on #17, this is failing

error: useless use of `vec!`
  --> src/ownership/file_generator.rs:26:9
   |
26 | /         vec![
27 | |             "# STOP! - DO NOT EDIT THIS FILE MANUALLY",
28 | |             "# This file was automatically generated by \"bin/codeownership validate\".",
29 | |             "#",
...  |
35 | |             "",
36 | |         ]
   | |_________^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_vec
   = note: `-D clippy::useless-vec` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(clippy::useless_vec)]`
help: you can use an array directly
   |
26 ~         ["# STOP! - DO NOT EDIT THIS FILE MANUALLY",
27 +             "# This file was automatically generated by \"bin/codeownership validate\".",
28 +             "#",
29 +             "# CODEOWNERS is used for GitHub to suggest code/file owners to various GitHub",
30 +             "# teams. This is useful when developers create Pull Requests since the",
31 +             "# code/file owner is notified. Reference GitHub docs for more details:",
32 +             "# https://help.github.com/en/articles/about-code-owners",
33 +             "",
34 +             ""]
   |

error: use of `or_insert` to construct default value
   --> src/ownership/validator.rs:158:45
    |
158 |                         let sources = entry.or_insert(Vec::new());
    |                                             ^^^^^^^^^^^^^^^^^^^^^ help: try: `or_default()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unwrap_or_default
    = note: `-D clippy::unwrap-or-default` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::unwrap_or_default)]`

error: use of `unwrap_or` to construct default value
   --> src/project.rs:227:79
    |
227 |                     owned_gems: deserializer.ruby.map(|ruby| ruby.owned_gems).unwrap_or(Vec::new()),
    |                                                                               ^^^^^^^^^^^^^^^^^^^^^ help: try: `unwrap_or_default()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unwrap_or_default

error: could not compile `codeowners` (bin "codeowners") due to 3 previous errors
Error: Process completed with exit code 101.

Maybe some of the workflows need to be pinned to specific versions? 🤔

[ethankhall]

We should add a rust-toolchain.toml to pin to a version of rust, so that builds will "just work" and not rely on local machine versions of rust installed.