storage manager: make rclone path configurable.

Fix bug where pruning uploads failed because the rclone binary couldn't
be found. The upload pruner runs under cron, which has a fixed default
$PATH of "/bin:/usr/bin", but in production rclone is installed under
/usr/local/bin. This caused the upload pruner to fail, which prevented
the rest of daily maintenance from running.

app/logical/storage_manager/rclone.rb

@@ -1,10 +1,11 @@
 class StorageManager::Rclone < StorageManager
   class Error < StandardError; end
-  attr_reader :remote, :bucket, :rclone_options
+  attr_reader :remote, :bucket, :rclone_path, :rclone_options
 
-  def initialize(remote:, bucket:, rclone_options: {}, **options)
+  def initialize(remote:, bucket:, rclone_path: "rclone", rclone_options: {}, **options)
     @remote = remote
     @bucket = bucket
+    @rclone_path = rclone_path
     @rclone_options = rclone_options
     super(**options)
   end
@@ -24,8 +25,8 @@ class StorageManager::Rclone < StorageManager
   end
 
   def rclone(*args)
-    success = system("rclone", *rclone_options, *args)
-    raise Error, "rclone #{$?}" if !success
+    success = system(rclone_path, *rclone_options, *args)
+    raise Error, "rclone #{args.join(" ")}: #{$?}" if !success
   end
 
   def key(path)