[CI] update main explicitly in long selftest job

• Requested reviewers: @pihlajakuja @norvell

The two recent runs of the long self test job were both on the Ericsson linux runner and revealed unexpected failures. On closer look, the main version that was used was outdated. After performing git checkout main, the new printout did not print the most recent commit on the main. I don't if that is happening because of some leftover main state from the last run or that the machine simply needs some time to notice the new version.

First approach for a solution: add an explicit git pull after checking out main. This would take care of the first possibility.