use pvc for pkg deps

.woodpecker/build.yaml

@@ -20,6 +20,10 @@ steps:
     pull: true
     environment:
       GIT_USER: pat-s
+      # set the location of the 'pkgcache' cache dir which persists the R package dependencies needed to install the packages themselves
+      R_PKG_CACHE_DIR: /mnt/cache/
+    volumes:
+      - arm-binaries-r-dep-cache:/mnt/cache
     commands:
       # download latest package version (avoid having to rebuild image every time)
       # rscript_startup arg: required to pass down global option to future workers https://github.com/HenrikBengtsson/future/issues/134#issuecomment-2245666169