ci: update actions/checkout and Swatinem/rust-cache actions

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CI failures are:

doctest failure: https://github.com/tokio-rs/async-backtrace/actions/runs/7439947852/job/20240413980

test backtrace/src/location.rs - location::location (line 7) ... FAILED

failures:

---- backtrace/src/location.rs - location::location (line 7) stdout ----
Test executable failed (exit status: 101).

stderr:
thread 'main' panicked at backtrace/src/location.rs:9:5:
assertion `left == right` failed
  left: 61
 right: 51

And new clippy warnings: https://github.com/tokio-rs/async-backtrace/actions/runs/7439947852/job/20240413550

error: this bound is already specified as the supertrait of `FusedIterator`
   --> backtrace/src/frame.rs:367:37
    |
367 |     pub fn backtrace(&self) -> impl Iterator<Item = &Frame> + FusedIterator {
    |                                     ^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#implied_bounds_in_impls
    = note: `-D clippy::implied-bounds-in-impls` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::implied_bounds_in_impls)]`
help: try removing this bound
    |
367 -     pub fn backtrace(&self) -> impl Iterator<Item = &Frame> + FusedIterator {
367 +     pub fn backtrace(&self) -> impl FusedIterator<Item = &Frame> {
    |

error: this bound is already specified as the supertrait of `FusedIterator`
   --> backtrace/src/frame.rs:402:51
    |
402 |     pub(crate) unsafe fn subframes(&self) -> impl Iterator<Item = &Frame> + FusedIterator {
    |                                                   ^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#implied_bounds_in_impls
help: try removing this bound
    |
402 -     pub(crate) unsafe fn subframes(&self) -> impl Iterator<Item = &Frame> + FusedIterator {
402 +     pub(crate) unsafe fn subframes(&self) -> impl FusedIterator<Item = &Frame> {
    |

The latter is easy to fix, but I'm not sure what the former is.