CI: conformance release build is 30-35s with warm cache - investigate

[iainmcgin]

The conformance CI jobs spend 30-35s on cargo build --release --bin ...
even with Swatinem/rust-cache@v2 restoring in 3-5s and the post-step
skipping save (meaning: cache hit, nothing new to write back).

From the PR #6 merge run:

Job	Cache restore	Build	Cache save
Conformance (server)	3s	30s	0s
Conformance (client)	5s	35s	0s

This was ~50s before #5 switched from building the whole
connectrpc-conformance package to --bin ${{ matrix.bin }}.

Hypothesis

Swatinem/rust-cache caches target/*/deps/ but not the final crate
artifacts - so dependencies are warm but connectrpc-conformance
itself recompiles from scratch every time. The conformance crate is
large (includes all the checked-in generated protobuf code), so 30s
of release-mode compile for the leaf crate might just be the floor.

If that's the answer, there's no fix - this is working as designed.
Filing this to record that someone should confirm it rather than
assume.

Possible levers if it's not the floor

• Check if LTO is on for the workspace release profile (slow link)
• Check if the codegen units setting is working against parallelism
• sccache as a second layer (caches per-crate compile output, not
  just deps)