Fixed bad windows temp file

Used filename:join to prevent duplicate slashes.
Acquired temp dir from mochitemp:gettempdir.

Signed-off-by: Alexander Shorin <kxepal@apache.org>
diff --git a/src/couch_plugins.erl b/src/couch_plugins.erl
index 0f7dbbe..c6dad48 100644
--- a/src/couch_plugins.erl
+++ b/src/couch_plugins.erl
@@ -180,7 +180,7 @@
 % downloads a pluygin .tar.gz into a local plugins directory
 -spec download(string()) -> ok | {error, string()}.
 download({Name, _BaseUrl, Version, _Checksums}=Plugin) ->
-  TargetFile = "/tmp/" ++ get_filename(Name, Version),
+  TargetFile = filename:join(mochitemp:gettempdir(), get_filename(Name, Version)),
   case file_exists(TargetFile) of
     %% wipe and redownload
     true -> file:delete(TargetFile);