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.

.crow/weekly-rebuild-missing.yaml

@@ -373,6 +373,8 @@ steps:
       R_LIBS_USER: /mnt/cache/R-pkgs
       R_VERSION: ${R_VERSION}
       CCACHE_DIR: /mnt/cache/ccache
+      # parallel make within one package build, matched to the step's CPU limit
+      MAKE_JOBS: 3
       PLATFORM: ${OS}
       ARCH: ${ARCH}
       NCPUS: 2
@@ -384,6 +386,7 @@ steps:
       REBUILD_BUDGET_HOURS: 20
     commands:
       - git clone -q https://pat-s:$$REPO_RO_TOKEN@git.devxy.io/devxy/build-cran-binaries.git .
+      - . local/compiler-cache.sh
       - mkdir -p /mnt/cache/uvr/cache /mnt/cache/uvr/packages /mnt/cache/R-pkgs /mnt/cache/ccache /mnt/cache/packages
       - rm -rf /mnt/cache/R-pkgs/00LOCK-*
       - /opt/R/$R_VERSION/bin/Rscript local/install-bincraft.R