Fix wrong severity of related errors

[dalance]

Currently, the severity of the parent error is shown as the severity of related errors.
This PR fix it.

Closes #232

[zkat]

oooh thanks for this!

[zkat]

looks like you just need some clippy fixes

[dalance]

Some clippy issues can't pass both 1.56.0 and stable.

• 1.56.0

error: there is no argument named `enum_name`
  --> miette-derive/src/url.rs:87:55
   |
87 |                         let item_path = format!("enum.{enum_name}.html#variant.{ident}");
   |

• stable

error: variables can be used directly in the `format!` string
  --> miette-derive/src/url.rs:87:41
   |
87 |                         let item_path = format!("enum.{}.html#variant.{}", enum_name, ident);
   |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninlined_format_args
   = note: `-D clippy::uninlined-format-args` implied by `-D warnings

[zkat]

Ah that's annoying. Let me look into making it so clippy only runs on stable, I guess. No point in running it on both anyway.

[dalance]

If you want to use 1.56.0 as MSRV, clippy should do at 1.56.0 only because new features which cause error at stable's clippy can't be used.

[zkat]

yeah, I guess you're right :)

[zkat]

b658fc0 should fix this hopefully

[zkat]

Looks like we're good now! Thanks again :)