apcupsd: unbreak build

Fix build since commit 093cc00cdd
("cc-wrapper: Always export environment variables for binutils").

The apcupsd build system uses "$(STRIP)" as flags for the "install"
program. Now that $STRIP is the path to the strip binary, we cannot pass
that to "install". Fixes this:

  ...
  COPY  multimon.cgi => /nix/store/...-apcupsd-3.14.14/libexec/cgi-bin
/nix/store/...-coreutils-8.27/bin/install: cannot stat 'strip': No such file or directory

pkgs/servers/apcupsd/default.nix

@@ -15,6 +15,11 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ pkgconfig utillinux man ] ++ stdenv.lib.optional enableCgiScripts gd;
 
+  prePatch = ''
+    sed -e "s,\$(INSTALL_PROGRAM) \$(STRIP),\$(INSTALL_PROGRAM)," \
+        -i ./src/apcagent/Makefile ./autoconf/targets.mak
+  '';
+
   # ./configure ignores --prefix, so we must specify some paths manually
   # There is no real reason for a bin/sbin split, so just use bin.
   preConfigure = ''