Build cryptography without build isolation in the tests session

uv's isolated build environment lives in a temporary directory with a
fresh random name. For non-abi3 builds (the free-threaded jobs)
maturin passes that interpreter to pyo3-build-config as PYO3_PYTHON,
which registers rerun-if-env-changed on it, so every CI run on 3.14t
and 3.15t-dev recompiled pyo3-ffi and pyo3 despite a warm cache, on
all three platforms. Building with the session's own interpreter gives
a stable path. The cache key suffix is bumped so the recorded
fingerprint is refreshed.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QZZQDQuPPD8T7jaAsCSHtM