Skip running clippy without default features (#10098)

## Problem

Running clippy with `cargo hack --feature-powerset` in CI isn't
particularly fast. This PR follows-up on
https://github.com/neondatabase/neon/pull/8912 to improve the speed of
our clippy runs.

Parallelism as suggested in
https://github.com/neondatabase/neon/issues/9901 was tested, but didn't
show consistent enough improvements to be worth it. It actually
increased the amount of work done, as there's less cache hits when
clippy runs are spread out over multiple target directories.
Additionally, parallelism makes it so caching needs to be thought about
more actively and copying around target directories to enable
parallelism eats the rest of the performance gains from parallel
execution.

After some discussion, the decision was to instead cut down on the
number of jobs that are running further. The easiest way to do this is
to not run clippy *without* default features. The list of default
features is empty for all crates, and I haven't found anything using
`cfg(feature = "default")` either, so this is likely not going to change
anything except speeding the runs up.

## Summary of changes

Reduce the amount of feature combinations tried by `cargo hack` (as
suggested in
https://github.com/neondatabase/neon/pull/8912#pullrequestreview-2286482368)
by never disabling default features.

## Alternatives

- We can split things out into different jobs which reduces the time
until everything is finished by running more things in parallel. This
does however decreases the amount of cache hits and increases the amount
of time spent on overhead tasks like repo cloning and restoring caches
by doing those multiple times instead of once.
- We could replace `cargo hack [...] clippy` with `cargo clippy [...];
cargo clippy --features testing`. I'm not 100% sure how this compares to
the change here in the PR, but it does seem to run a bit faster. That
likely means it's doing less work, but without understanding what
exactly we loose by that I'd rather not do that for now. I'd appreciate
input on this though.

.github/workflows/build_and_test.yml

@@ -212,7 +212,7 @@ jobs:
           fi
           echo "CLIPPY_COMMON_ARGS=${CLIPPY_COMMON_ARGS}" >> $GITHUB_ENV
       - name: Run cargo clippy (debug)
-        run: cargo hack --feature-powerset clippy $CLIPPY_COMMON_ARGS
+        run: cargo hack --features default --ignore-unknown-features --feature-powerset clippy $CLIPPY_COMMON_ARGS
 
       - name: Check documentation generation
         run: cargo doc --workspace --no-deps --document-private-items