apcupsd: unbreak build

Fix build since commit 093cc00c
("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