Systemtest summary not properly escaped

[fsimonis]

The systemtest workflows add commit messages to the summaries.
These commit messages are not escaped though.
This leads to backticks in the commit messages being interpreted as subshells, leading to various problems.

https://github.com/precice/precice/actions/runs/18374238279/job/52345314741

[fsimonis]

Using single quotes seems to work. precice/precice@59f0644