fix(install): generate /etc/machine-id on systems without /proc (#2274)

* fix(install): generate /etc/machine-id on systems without /proc

The install script read the fingerprint UUID from
/proc/sys/kernel/random/uuid, which FreeBSD does not have. On pfSense
the redirect still created /etc/machine-id, cat failed, and the agent
was left with an empty machine-id file. The [ ! -f ] guard then skipped
regeneration on every later run.

Fall back to uuidgen when /proc is unavailable, mirroring how the
script already picks between sha256sum and FreeBSD's sha256, and leave
no file behind when neither source exists.

* fix(install): regenerate empty machine id

supplemental/scripts/install-agent.sh

@@ -796,9 +796,16 @@ set_selinux_context
 # Cleanup
 rm -rf "$TEMP_DIR"
 
-# Make sure /etc/machine-id exists for persistent fingerprint
-if [ ! -f /etc/machine-id ]; then
-  cat /proc/sys/kernel/random/uuid | tr -d '-' > /etc/machine-id
+# Make sure /etc/machine-id exists and is non-empty for persistent fingerprint
+if [ ! -s /etc/machine-id ]; then
+  if [ -r /proc/sys/kernel/random/uuid ]; then
+    tr -d '-' < /proc/sys/kernel/random/uuid > /etc/machine-id
+  elif command -v uuidgen >/dev/null; then
+    # FreeBSD has no /proc/sys/kernel/random/uuid
+    uuidgen | tr -d '-' > /etc/machine-id
+  else
+    echo "No UUID source found, skipping /etc/machine-id creation"
+  fi
 fi
 
 # Check for NVIDIA GPUs and grant device permissions for systemd service