ci: avoid another date-dependent failure

Set the day before setting the month to avoid failure when running the test on the 31st day of a month.

This is similar to f4b7f5de

Test Case

Set your system calendar to the 31st day of the month (for instance July 31)

cd testsuite/gtk
gcc -o notify notify.c $(pkgconf --cflags --libs gtk4 gtk4-unix-print)
./notify

The test should complete successfully like it does if it is not the 31st day of the month.

Other Info

This was detected by Debian build tests: https://buildd.debian.org/status/logs.php?pkg=gtk4&arch=amd64