Feature: INSTALL_PREFIX to relocate the agent installation

[NiceGuyIT]

INSTALL_PREFIX relocates the agent's install tree to a writable root so the agent can run on systems where /etc, /opt, and /usr are read-only. Some systems, such as TrueNAS Scale, will have a read-only root and expect custom programs to run in a mounted "system" filesystem.

The installation can be moved by setting one of the following.

• --install-prefix <path> CLI flag; takes precedent over $INSTALL_PREFIX.
• $INSTALL_PREFIX environment variable

Relates to PR #80 in the rmmagent repo.

Fixes this comment.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.87%. Comparing base (72b6a73) to head (e269ca9).
⚠️ Report is 112 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2462      +/-   ##
===========================================
+ Coverage    74.44%   75.87%   +1.42%     
===========================================
  Files          151      151              
  Lines        11996    11608     -388     
===========================================
- Hits          8931     8808     -123     
+ Misses        3065     2800     -265     

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[wh1te909]

thank you, I will try to get this in the release after next (current release is already frozen and QA just finished)