libcap: Fix static build setting LIBCSTATIC=yes

Fixes the build of pkgsStatic.libcap, as it was trying to link to the
dinamic libc. From libcap-2.69/progs/Makefile:

...
ifeq ($(LIBCSTATIC),yes)
LDFLAGS = --static
DEPS = ../libcap/libcap.a
else
# For this build variant override the LDFLAGS to link statically from
# libraries within the build tree. If you never want this, use make
# DYNAMIC=yes . Note, we can't reliably link statically against glibc
# becasuse of https://sourceware.org/bugzilla/show_bug.cgi?id=12491 .
LDFLAGS = -Wl,-Bstatic
LDFLAGS_SUFFIX = -Wl,-Bdynamic
DEPS = ../libcap/libcap.a
endif
...

pkgs/os-specific/linux/libcap/default.nix

···
     "BUILD_CC=$(CC_FOR_BUILD)"
     "CC:=$(CC)"
     "CROSS_COMPILE=${stdenv.cc.targetPrefix}"
-  ] ++ lib.optional isStatic "SHARED=no";
+  ] ++ lib.optionals isStatic [ "SHARED=no" "LIBCSTATIC=yes" ];
 
   postPatch = ''
     patchShebangs ./progs/mkcapshdoc.sh