🔧 Add cargo-about to the rust-tools prebuilt loop 🔧

Adds cargo-about (third-party license report/notice generation from
Cargo.lock) to the toolchain — it was previously absent from the tool
list entirely, not broken or missing a prebuilt. Confirmed musl prebuilts
exist for both x86_64 and aarch64 via the GitHub releases API before
placing it in the main prebuilt-fetch loop (not the compile-fallback one).

- Dockerfile: added cargo-about to the rust-tools stage's main cargo
  binstall loop, grouped with the other license/SBOM tools
  (cargo-audit, cargo-deny, cargo-cyclonedx).
- README.md: documented cargo-about in the linting & static analysis
  tool table.

Dockerfile

@@ -110,6 +110,7 @@ RUN --mount=type=cache,id=cargo-registry-native,sharing=shared,target=/usr/local
       cargo-expand \
       bacon \
       cargo-audit \
+      cargo-about \
       cargo-deny \
       cargo-cyclonedx \
       cargo-machete \