tools/ci.sh: Run natmod tests as part of CI for Unix/RV64.

This commit updates the test procedure for the Unix port targeting the
RV64 platform to also run the battery of native modules test that is
part of the QEMU port.

Unfortunately this required a few changes to the CI setup since the Unix
port was still using an older version of Ubuntu LTS than the RISC-V
natmods build infrastructure expects.  Updating the OS version just for
the RV64 Unix target brought a couple of issues when building the code
(an extra package is now needed to let FFI build) and running tests
(QEMU binfmt support requires a new setup).

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

.github/workflows/ports_unix.yml

@@ -277,8 +277,7 @@ jobs:
       run: tests/run-tests.py --print-failures
 
   qemu_riscv64:
-    # ubuntu-22.04 is needed for older libffi.
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v6
     - name: Install packages