run_vim.sh: use explicit HOME in /tmp (#11)

This is required for Neovim to suppress errors that it cannot write to
~/.local etc.

Author: blueyed

scripts/run_vim.sh

@@ -21,4 +21,5 @@ done
 
 # Run as the vimtest user.  This is not really for security.  It is for running
 # Vim as a user that's unable to write to your volume.
-exec su -l vimtest -c "cd /testplugin && /vim-build/bin/$BIN $ARGS"
+cd /testplugin || exit
+exec su -l vimtest -c "env HOME=/tmp/vimtestbed-home /vim-build/bin/$BIN $ARGS"