create_tarball: read $debug from environment

If $debug is set in the environment, use that.  This allows enabling
debug mode without requiring an edit to the script.

Signed-off-by: Jeff Squyres <[email protected]>

Author: jsquyres

contrib/nightly/make_snapshot_tarball

@@ -35,7 +35,7 @@ gitbranch=$5
 
 # Set this to any value for additional output; typically only when
 # debugging
-debug=
+: ${debug:=}
 
 # do you want a success mail?
 want_success_mail=1