mkosi: Set meson --max-lines= to 300 in CI

By default meson only shows the last 100 lines of output for failed
tests. Let's bump this to 300 with the new --max-lines= option I added
so we get more useful output on test failures.

(cherry picked from commit e469add)

Author: DaanDeMeyer

.github/workflows/mkosi.yml

@@ -202,7 +202,8 @@ jobs:
             --suite integration-tests \
             --print-errorlogs \
             --no-stdsplit \
-            --num-processes "$(($(nproc) - 1))"
+            --num-processes "$(($(nproc) - 1))" \
+            --max-lines 300
 
       - name: Archive failed test journals
         uses: actions/upload-artifact@v4