CI: tolerate missing artifacts

When a VM fails to launch or is unreachable the qemu-7-prepare.sh
script will fail to collect the artifacts due to the missing vm*
directories.  We want to collect as much diagnostic information as
possible, when missing create the directory to allow the subsequent
steps to proceed normally.  Additionally, we don't want to fail
if the /tmp/summary.txt file is missing.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #18438

.github/workflows/scripts/qemu-8-summary.sh

@@ -37,9 +37,11 @@ function showfile_tail() {
   echo "##[endgroup]"
 }
 
-# overview
-cat /tmp/summary.txt
-echo ""
+# overview if available
+if [ -f /tmp/summary.txt -a -s /tmp/summary.txt ]; then
+  cat /tmp/summary.txt
+  echo ""
+fi
 
 if [ -f /tmp/have_failed_tests -a -s /tmp/failed.txt ]; then
   echo "Debuginfo of failed tests:"