nixos/apcupsd: don't evaluate event hooks at build time

Better replace the double quotes in 'echo "${commands}"' with single
quotes, to prevent the shell from doing command substitution etc. at
configuration build time.

nixos/modules/services/monitoring/apcupsd.nix

@@ -39,7 +39,7 @@ let
 
   shellCmdsForEventScript = eventname: commands: ''
     echo "#!${pkgs.stdenv.shell}" > "$out/${eventname}"
-    echo "${commands}" >> "$out/${eventname}"
+    echo '${commands}' >> "$out/${eventname}"
     chmod a+x "$out/${eventname}"
   '';