Linux QNN Pipeline: fix build error reporting (#14922)
### Description
Split up the ORT build step in the Linux QNN CI Pipeline.
### Motivation and Context
Build errors were not being immediately reported at the end of the build
step. The build step currently concatenates multiple shell commands, and
the return code for the last (mkdir) was being reported. This PR ensures
that the return code of the `python build.py ...` command is reported
for the build step.
