toolbox.sh: Don't mask realpath's return value

toolbox.sh: Don't mask realpath's return value

When determining the location of toolbox.sh, the result from realpath
is passed to dirname to remove the script name from the path. However,
both commands are executed in the same statement, causing the return
value of realpath to be masked by dirname.

This commit splits the statement into separate steps, so that return
values are not masked.