packetbeat/beater: help the GC clean up the Npcap installer if it's n…

…ot used (#30513)

The installer bytes are released in the install case but this was accidentally
removed in #30048. This frees ~1MB of runtime memory after start up has been
completed.

(cherry picked from commit d76e445)

packetbeat/beater/install_npcap.go

@@ -52,6 +52,7 @@ func installNpcap(b *beat.Beat) error {
 		}
 	}()
 	if !npcap.Upgradeable() {
+		npcap.Installer = nil
 		return nil
 	}