Fix ENGINX data cache search on IDAaaS

[GuiMacielPereira]

Description of work

From the issue described in #37398, the ENGINX files were still not being found in the data cache because part of the path was still using ENGIN-X instead of ENGINX.

Summary of work

I changed a few lines of code that ensure that when the shortname of the instrument is used (ENGINX) to find the index file, the path is also constructed consistently with this, and uses the shortname version.

Access ENGINX files easily on IDAaaS from the data cache.
Fixes #37398 .

Further detail of work

To test:

Test with build number 892.

1. Install mantid python package (workbench not needed) with conda install -c guimacielpereira/label/fix-enginx mantid
2. Open python interpreter on terminal by typing python
3. Run:

from mantid.simpleapi import Load
Load("ENGINX283000")

4. Should get an error with permissions denied, but the path it tried to access should be printed
5. The important bit is that the path shows the file was trying to be accessed inside the folder ENGINX instead of ENGIN-X

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[jhaigh0]

Load is now looking in the correct place. With Gui's build I got this permission denied error upon trying to load the data.

This shows that Load was searching in the correct place /data/instrument/ENGINX

[cailafinn]

Code and the newly added test look good.

Does this need a release note before merging? If I'm reading the history right this would have been a problem in 6.10.

[cailafinn]

Just noticed a typo in the release note.