Allow using ExtraSearchPaths= with ToolsTree=

Allow using ExtraSearchPaths= with ToolsTree=

If we find a binary in any path configured with ExtraSearchPaths=,
we prefer using it over the tools tree.

This commit also gets rid of all the find_binary() for "btrfs" in
tree.py and stops passing in the tools tree to those functions.
Instead, we rely on the new behavior of run() to not fail hard
when check=False and the btrfs binary is not found.