rand: appease clang-12 and later

[mrc0mmand]

[11/28] Compiling C object dfuzzer.p/src_rand.c.o
../src/rand.c:541:17: warning: suspicious concatenation of string literals in an array initialization; did you mean to separate the elements with a comma? [-Wstring-concatenation]
                "%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n",
                ^
../src/rand.c:540:17: note: place parentheses around the string literal to silence warning
                "%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n%n" \
                ^
1 warning generated.

Reported in https://github.com/dbus-fuzzer/dfuzzer/pull/63#issuecomment-1731324397

---

Also, let's switch all CI jobs to use the ubuntu-latest images, so we don't have to bump them manually (and so we're not stuck with old stuff when we forgot, like we did with the build job).

[evverx]

I have to admit I considered removing this string (or at least shortening it) to make both clang and coverity happy but it's just so weirdly specific that I couldn't do it :-)

LGTM. Thanks!