fix(libnpmexec): workspaces support

Fixes the proper path location to use when targetting specific
workspaces.

Makes it so that libnpmexec is always reading from the
Arborist actual tree instead of reading `node_modules` from
the file system when retrieving local package data.

Fixes: npm#3520
Relates to: npm#4619
Relates to: npm/statusboard#403

Author: ruyadorno

lib/commands/exec.js

@@ -95,7 +95,7 @@ class Exec extends BaseCommand {
 
     for (const path of this.workspacePaths) {
       const locationMsg = await getLocationMsg({ color, path })
-      await this.exec(args, { locationMsg, path, runPath: path })
+      await this.exec(args, { locationMsg, path: this.npm.localPrefix, runPath: path })
     }
   }
 }