Task: Fix vcpkg caching (x-gha removed upstream)

Restore vcpkg binary caching across all five CI workflows after the x-gha backend was silently removed upstream.

• x-gha backend replaced with files provider backed by actions/cache@v4.
• All 5 workflows updated: canary-linux, continuous-linux, continuous-windows, continuous-macos, nightly-linux.
• First run populates the cache; subsequent runs drop vcpkg install from ~21 min to seconds.

Captured during execution; cleared into the parent story on close.

Silent upstream removal meant every CI run was rebuilding 120 packages from source. The cache hit-rate is the load-bearing piece of CI speed.

CI back to expected times; no more 21-min vcpkg installs.