Ensure UnwindSafe even with "backtrace" feature enabled and old Rust

[dtolnay]

On Rust version 1.72 and older, this code didn't used to compile:

// [dependencies]
// anyhow = { version = "1", features = ["backtrace"] }

fn main() {
    let err = anyhow::Error::msg("...");
    let _ = std::panic::catch_unwind(move || err);
}

error[E0277]: the type `UnsafeCell<backtrace::Capture>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
   --> src/main.rs:6:38
    |
6   |     let _ = std::panic::catch_unwind(move || err);
    |             ------------------------ ^^^^^^^^^^^ `UnsafeCell<backtrace::Capture>` may contain interior mutability and a reference may not be safely transferrable across a catch_unwind boundary
    |             |
    |             required by a bound introduced by this call
    |
    = help: within `anyhow::error::ErrorImpl`, the trait `RefUnwindSafe` is not implemented for `UnsafeCell<backtrace::Capture>`
note: required because it appears within the type `LazilyResolvedCapture`
   --> ~/.rustup/toolchains/1.72-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/backtrace.rs:433:8
    |
433 | struct LazilyResolvedCapture {
    |        ^^^^^^^^^^^^^^^^^^^^^
note: required because it appears within the type `Inner`
   --> ~/.rustup/toolchains/1.72-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/backtrace.rs:133:6
    |
133 | enum Inner {
    |      ^^^^^
note: required because it appears within the type `Backtrace`
   --> ~/.rustup/toolchains/1.72-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/backtrace.rs:109:12
    |
109 | pub struct Backtrace {
    |            ^^^^^^^^^
note: required because it appears within the type `Option<Backtrace>`
   --> ~/.rustup/toolchains/1.72-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/option.rs:563:10
    |
563 | pub enum Option<T> {
    |          ^^^^^^
note: required because it appears within the type `ErrorImpl`
   --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/anyhow-1.0.88/src/error.rs:863:19
    |
863 | pub(crate) struct ErrorImpl<E = ()> {
    |                   ^^^^^^^^^
    = note: required for `NonNull<anyhow::error::ErrorImpl>` to implement `UnwindSafe`
note: required because it appears within the type `Own<ErrorImpl>`
   --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/anyhow-1.0.88/src/ptr.rs:6:12
    |
6   | pub struct Own<T>
    |            ^^^
note: required because it appears within the type `Error`
   --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/anyhow-1.0.88/src/lib.rs:389:12
    |
389 | pub struct Error {
    |            ^^^^^
note: required because it's used within this closure
   --> src/main.rs:6:38
    |
6   |     let _ = std::panic::catch_unwind(move || err);
    |                                      ^^^^^^^
note: required by a bound in `catch_unwind`
   --> ~/.rustup/toolchains/1.72-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/panic.rs:141:40
    |
141 | pub fn catch_unwind<F: FnOnce() -> R + UnwindSafe, R>(f: F) -> Result<R> {
    |                                        ^^^^^^^^^^ required by this bound in `catch_unwind`