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