ci: drop the docker layer cache and stop installing unused tooling (#5544)

Two cleanups, neither changing what is built or published.

docker.yml loses its type=gha layer cache. The image build compiles nothing —
it downloads a release zip and runs apk/apt — so the cache could only save the
minute or two those take, against a real correctness problem: with
RELEASE=latest the binary URL is resolved by curl inside a RUN layer, and the
layer key does not include what it resolved to. A rebuild at the same RELEASE
value (a dispatch with version=latest, or a re-run of the same version) would
hit the old layer and ship the previous release's binary. mode=max also drew on
the same repo-wide 10GB Actions cache quota the Rust lanes are contending for.

Only RELEASE is passed as a build-arg now; it is the sole one the Dockerfiles
declare besides TARGETARCH. BUILDTIME, VERSION, BUILD_TYPE, REVISION and CHANNEL
were read by no stage, and BUILDTIME's $(date ...) was a literal string in the
YAML block rather than a substitution. The DOCKER_CHANNEL computation that fed
CHANNEL evaluated to "release" down every branch and is gone with it. BUILD_DATE
and VCS_REF stay unset even though the Dockerfiles declare them: supplying them
would change the published image labels.

The setup composite stops installing tools its callers do not use. protobuf-
compiler comes out of the apt list entirely — setup-protoc installs 34.1 into
the tool cache and prepends it to PATH, so the apt build was shadowed on every
run and never used; the same duplicate install is removed from the io_uring lane.
musl-tools, zip and unzip move behind install-build-packaging-tools, off for the
CI and coverage lanes and left on for build.yml, whose native musl leg needs
musl-gcc and whose packaging steps need zip. cargo-nextest and the
rustfmt/clippy components move behind install-test-tools, off only for build.yml,
which runs no tests and no lints; coverage and the nightly replication lane keep
them because both invoke nextest.

The action's github-token input is deleted along with its 20 call sites. Its
runs block never referenced it — setup-protoc uses github.token directly — so it
was 20 places handing a token to something that ignored it.

Refs: rustfs/backlog#1598, rustfs/backlog#1600, rustfs/backlog#1603

.github/workflows/build.yml

@@ -265,7 +265,6 @@ jobs:
           rust-version: stable
           target: ${{ matrix.target }}
           cache-shared-key: build-${{ matrix.target }}
-          github-token: ${{ secrets.GITHUB_TOKEN }}
           # main only. A cache saved on refs/tags/X is scoped to that tag: no
           # other tag, no main run and no PR can restore it, so every release
           # cycle wrote up to 12 entries of 1-2GB (preview tag plus final tag,
@@ -276,6 +275,7 @@ jobs:
           # finds that tag's own warm cache and falls back to main's.
           cache-save-if: ${{ github.ref == 'refs/heads/main' }}
           install-cross-tools: ${{ matrix.cross }}
+          install-test-tools: 'false'
 
       - name: Download static console assets
         shell: bash