ci: add ccache to libuvc build check

[andrescera]

What

Add bounded ccache coverage to the libuvc CMake build-check workflow and document the cache contract.

Why

The w6-6 audit found that libuvc's Ubuntu C/C++ builds had no compiler cache. The workflow now installs ccache, restores/saves only a workspace-local ccache directory, keys it by runner/compiler/source inputs, and wires both auto-detach ON/OFF CMake configurations through ccache.

How to verify

• actionlint 1.7.12 passes for .github/workflows/build.yml.
• Both CMake variants build shared and static libraries locally.
• Same-path clean rebuild reports 36/36 direct ccache hits with a 200 MB bound.
• CTest exits 0; this repository has no registered CTest tests.
• git diff --check and the scoped diff audit pass; no tests were changed, deleted, skipped, or weakened.

Risks

Only compiler-cache files are cached; build/, build-off/, and generated artifacts are excluded. The documentation-only web.yml workflow is unchanged.