Follow-up fix to 3e2a4eef (re #11109)

Using `:` as sed-`s`-expr separator is not a good idea on windows, as
this can result in an expression like

   's:@INSTALL@:$(subst ../install-sh,E:/msys64/home/foo/ghc/install-sh,@INSTALL@):g'

(note the `E:/` drive component)

Using `|` instead of `:` has less risk of collision with the substitution expression

libffi/ghc.mk

@@ -82,7 +82,7 @@ $(libffi_STAMP_CONFIGURE): $(TOUCH_DEP)
 	# install-sh is used when /usr/bin/install is missing; ensure its
 	# path in libffi's Makefile is correct. See GHC #11109.
 	mv libffi/build/Makefile.in libffi/build/Makefile.in.orig
-	sed 's:@INSTALL@:$$(subst ../install-sh,$(TOP)/install-sh,@INSTALL@):g' < libffi/build/Makefile.in.orig > libffi/build/Makefile.in
+	sed 's|@INSTALL@|$$(subst ../install-sh,$(TOP)/install-sh,@INSTALL@)|g' < libffi/build/Makefile.in.orig > libffi/build/Makefile.in
 
 # * Because -Werror may be in SRC_CC_OPTS/SRC_LD_OPTS, we need to turn
 #   warnings off or the compilation of libffi might fail due to warnings;