perf(ci): route compiles through ccache and use parallel make

Prepend ccache's masquerade dir to PATH in the build pipelines and set the ccache options that make hits possible for R's per-package temp dirs, plus MAKEFLAGS matched to each step's CPU limit.

local/compiler-cache.sh

@@ -0,0 +1,32 @@
+# Sourced (`. local/compiler-cache.sh`) by the build pipelines before any
+# package is compiled. POSIX sh: the Alpine images run steps under busybox.
+#
+# Routes gcc/g++ through ccache. The images name their compilers by bare
+# `gcc`/`g++` in both R's Makeconf and /root/.R/Makevars, so prepending ccache's
+# masquerade dir catches every compile, CMake-driven vendored builds included.
+# Wrapping CC in a Makevars instead would need R_MAKEVARS_USER, which replaces
+# the images' /root/.R/Makevars (CXX_STD, the musl defines) rather than adding
+# to it. The dir differs per distro: Alpine, RHEL, Debian/Ubuntu.
+for dir in /usr/lib/ccache/bin /usr/lib64/ccache /usr/lib/ccache; do
+  if [ -x "$dir/gcc" ]; then
+    PATH="$dir:$PATH"
+    break
+  fi
+done
+export PATH
+
+# R compiles every package in a fresh /tmp/RtmpXXXXXX/<pkg> dir. R's default
+# flags include -g, which makes ccache hash the working directory, and the line
+# markers carry the absolute source path, so without these two settings no
+# compile ever hits the cache across runs.
+export CCACHE_BASEDIR="${CCACHE_BASEDIR:-/tmp}"
+export CCACHE_NOHASHDIR=1
+# The images are rebuilt regularly, which changes the compiler's mtime but not
+# its content. ccache's default mtime check would discard the cache each time.
+export CCACHE_COMPILERCHECK=content
+
+# Parallel make inside a single package build. Match it to the step's CPU limit
+# via MAKE_JOBS; an explicit MAKEFLAGS wins.
+export MAKEFLAGS="${MAKEFLAGS:--j${MAKE_JOBS:-2}}"
+
+echo "compiler-cache: $(command -v gcc), CCACHE_DIR=${CCACHE_DIR:-unset}, MAKEFLAGS=$MAKEFLAGS"