builder.py doesn't calculate free vs required disk space correctly

[ConanMishler]

builder.py gives the user the option to specify a custom directory on which to mount an EXR service which required a mount volume. Unfortunately, it doesn't recognize when the specified custom directory is actually mounted on the same Linux volume as other EXR services. Therefore, its calculations for required disk space are incorrect. For example:

In this run of builder.py, the directory /bigdrive was specified for the UTXO_PLUGIN service. However, when calculations were made to determine total available space vs total required space on each of the specified mount directories, builder.py failed to recognize that /bigdrive is actually located on the same root / volume as /snode; therefore, the total disk space required on /bigdrive should be summed with the total disk space required on /snode, then that sum should be compared against the total available disk space on /.