Preserve rustup-managed files in Ubuntu runner images

[reaperhulk]

Updating the stable Rust toolchain in the Ubuntu armv7 runner fails while uninstalling Cargo because the image deleted share/man/man1/cargo.1, which is still recorded in rustup’s component manifest. This blocks the Thumb stable job in verified-garbage #453 before compilation (failure log).

Preserve the Rust documentation/manpages, LLVM tools, sanitizer runtimes, and wasm linker recorded in rustup’s manifests. Keep binary stripping and libLLVM deduplication. This increases image size but allows rustup to uninstall and update the baked-in toolchain correctly.

Enable rustup’s RUSTUP_PERMIT_COPY_RENAME fallback so toolchain updates can move directories on Docker overlay filesystems. Without it, the ppc64le runner fails with Invalid cross-device link while backing up share/doc/cargo.

Add an Ubuntu image CI check that removes Cargo, LLVM tools, rustc, and rust-std in a disposable container. This exercises the failure path deterministically even when stable has not changed since the image was built. PR builds test the locally built image; main builds pull the newly published image rather than testing the previous cached tag.

Validation:

• Reproduced the exact missing cargo.1 failure with rustup 1.29.1 after deleting manpages from an isolated macOS toolchain copy.
• Removing all four components succeeds from an isolated copy with files preserved.
• Dockerfile Rust installation shell syntax, workflow validation (actionlint -shellcheck=), and git diff --check pass. Full actionlint reports a pre-existing SC2086 warning in the Node version setup step.
• Docker is unavailable locally; Linux image builds and the new component-uninstall check will run in this PR’s CI.