Move build.sh execution from %posttrans to rockstor-build.service

[phillxnet]

We have likely already stretched our use of the rpm packaging systems %posttrans for our venv, initial secrets setup, and jslibs preparation, via the build.sh rockstor-core bash script. Move the execution of build.sh from outside of the constraints of the packaging system, to the level of OS orchestration: systemd.

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Partner issue to:

• rockstor-core's: "Add rockstor-build systemd service": https://github.com/rockstor/rockstor-core/issues/2793
• rockstor-installer: "Tumbleweed: build.sh fails as no /etc/locale.conf": https://github.com/rockstor/rockstor-installer/issues/156

Partner PR:

• rockstor-core's: "Add rockstor-build systemd service ...": https://github.com/rockstor/rockstor-core/pull/2795

[phillxnet]

Closing as: Fixed by #63