ci(tests): Install the binding wheels only for the soak tier
No integration or e2e suite drives pixelflux or pcmflux through their
Python bindings any more: the server, selkies gpu-probe, the virtual
camera example, the suites' Rust helpers and the crates' own Rust
tests stand in. Only the soak tier, the bindings' own API suites,
imports them.
So tests.yaml builds the pixelflux and pcmflux wheels for a standalone
run, and the suites job downloads and installs them, only where the
soak tier runs. ci.yaml's unit/integration suites and E2E smoke no
longer wait for the binding wheels, which it still builds for main's
pre-release.