[SLE15-SP5] Fixed ERB template loading in self update (bsc#1219174)

[lslezak]

Problem

• https://bugzilla.suse.com/show_bug.cgi?id=1219174 - YaST crashes after applying self updates
• The self update process overrides only the modified files to save memory in the inst-sys.
• The modified files are placed into /mounts/yast_0000/... subdirectory
• But as Ruby uses /mounts/yast_0000/... path for the __dir__ value in the Ruby file then the relative ERB template is not found because it was not modified and still lives in the original location at /usr/share/....

Solution

• If the template cannot be found using a relative path then fallback to the absolute path

Testing

• Tested manually, the crash is fixed

[coveralls]

coverage: 37.405% (-0.001%) from 37.406%
when pulling ace4381 on fix_erb_loading
into 72fb7ff on SLE-15-SP5.

[yast-bot]

✔️ Internal Jenkins job #754 successfully finished
✔️ Created IBS submit request #320946