Keep trimming when pct fstrim fails on one container

pct fstrim exits 255 when the container is locked, for example while a
backup runs ("CT is locked (backup)"). Under errexit the command
substitution ended the whole run there, before the output check below it
and before the remaining containers, and telemetry filed it as
"DPKG: Fatal internal error". Print the result and go on instead.

tools/pve/fstrim.sh

@@ -118,7 +118,8 @@ function trim_container() {
   fi
 
   local fstrim_output
-  fstrim_output=$(pct fstrim "$container" 2>&1)
+  # pct fstrim fails on a locked CT (e.g. a running backup); report it and go on.
+  fstrim_output=$(pct fstrim "$container" 2>&1) || true
   if echo "$fstrim_output" | grep -qi "not supported"; then
     echo -e "${RD}fstrim isnt supported on this storage!${CL}"
   elif echo "$fstrim_output" | grep -Eq '([0-9]+(\.[0-9]+)?\s*[KMGT]?B)'; then