ci: rework sccache for cross-rs/cross

.cross/Dockerfile

@@ -3,10 +3,31 @@ ARG DEBIAN_FRONTEND=noninteractive
 
 FROM $CROSS_BASE_IMAGE
 
+# Install system deps
+COPY .github/sccache.sh /
 RUN apt-get update && \
-    apt-get install -y --no-install-recommends clang llvm && \
+    apt-get install -y --no-install-recommends \
+      clang llvm curl ca-certificates && \
+    chmod +x /sccache.sh && \
+    arch=$(uname -m) && \
+    case "$arch" in \
+      x86_64)  triple="x86_64-unknown-linux-musl" ;; \
+      aarch64) triple="aarch64-unknown-linux-musl" ;; \
+      armv7l|armv7) triple="armv7-unknown-linux-musleabi" ;; \
+      i686|i586) triple="i686-unknown-linux-musl" ;; \
+      *) echo "FATAL: unsupported architecture for sccache: $arch" && exit 1 ;; \
+    esac && \
+    echo "Detected arch=$arch, downloading sccache for $triple" && \
+    /sccache.sh "$triple" && \
+    apt-get purge -y curl && \
+    apt-get autoremove -y && \
     rm -rf /var/lib/apt/lists/*
 
 ENV CC=clang \
     CXX=clang++ \
-    AR=llvm-ar
+    AR=llvm-ar \
+    RUSTC_WRAPPER="/usr/bin/sccache" \
+    SCCACHE_DIRECT=true \
+    SCCACHE_SERVER_UDS="/tmp/sccache.sock" \
+    SCCACHE_DIR="/tmp/sccache" \
+    CARGO_BUILD_PIPELINING=false

.github/sccache.sh

@@ -0,0 +1,30 @@
+#!/bin/bash
+#
+# Install sccache from GitHub releases.
+# Assumes curl is already installed in the image.
+# Usage: sccache.sh <target-triple>
+#   e.g. sccache.sh x86_64-unknown-linux-musl
+
+set -x
+set -euo pipefail
+
+main() {
+    local triple="${1}"
+    local version="v0.14.0"
+    local url="https://github.com/mozilla/sccache/releases/download/${version}/sccache-${version}-${triple}.tar.gz"
+    local td
+
+    td="$(mktemp -d)"
+    pushd "${td}"
+
+    curl -LSfs "${url}" -o sccache.tar.gz
+    tar -xzf sccache.tar.gz
+    cp "sccache-${version}-${triple}/sccache" "/usr/bin/sccache"
+    chmod +x "/usr/bin/sccache"
+
+    popd
+    rm -rf "${td}"
+    rm -f "${0}"
+}
+
+main "${@}"

Cross.toml

@@ -1,8 +1,8 @@
 [build.env]
 passthrough = ["RUSTFLAGS", "RUSTC_BOOTSTRAP"]
+volumes = ["/tmp=/tmp"]
 
 # Use clang as the C/C++ compiler to avoid the aws-lc-sys GCC memcmp bug
 # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95189
-# The custom Dockerfile installs clang/llvm and sets CC=clang, CXX=clang++, AR=llvm-ar
 [build.dockerfile]
-file = ".cross/Dockerfile"
+file = ".cross/Dockerfile"