Add timing metrics to prune step (#527)

lib/dependencies.sh

@@ -107,8 +107,10 @@ yarn_prune_devdependencies() {
     echo "Skipping because YARN_PRODUCTION is not 'true'"
     return 0
   else 
+    local start=$(nowms)
     cd "$build_dir" 
     yarn install --frozen-lockfile --ignore-engines --ignore-scripts --prefer-offline 2>&1
+    mtime "prune.yarn.time" "${start}"
   fi
 }
 
@@ -170,7 +172,9 @@ npm_prune_devdependencies() {
     echo "https://github.com/npm/npm/issues/17781"
     return 0
   else
+    local start=$(nowms)
     cd "$build_dir" 
     npm prune --userconfig $build_dir/.npmrc 2>&1
+    mtime "prune.npm.time" "${start}"
   fi
 }