Fixed yupdate script (bsc#1213197)

[lslezak]

Problem

• Patching the Agama installer does not work properly
• It fails because the pre/post scripts are not executed

Solution

• It turned out that the GitHub tarballs have all content in a <repo>-<branch> subdirectory, e.g. agama-master
• When downloading from GitHub add this extra path so the files are correctly found

Testing

• Tested manually

[coveralls]

coverage: 40.807% (-0.004%) from 40.811% when pulling cb38a5eefb0f0fc4c900fee51977072b47c1a2b7 on yupdate_fix into 44200d2f3c0b5bed255ecb8f1debde54ab618bf1 on master.

[yast-bot]

:heavy_check_mark: Public Jenkins job #255 successfully finished :heavy_check_mark: Created OBS submit request #1098142