Fix Windows installation directory in tools/ci-build

This commit sets the installation directory for the Windows build to the
same value as in the config/Makefile.m* files.

So, in itself this commit is a no-op. It however introduces the next one
which will ensure ci-build's instdir setting is honored even for
the builds that do not use configure.

tools/ci-build

@@ -109,22 +109,22 @@ case "${OCAML_ARCH}" in
   cygwin)
     cleanup=true;;
   mingw)
-    instdir=/cygdrive/c/ocamlmgw
+    instdir='C:/ocamlmgw'
     configure=nt
     cleanup=true
   ;;
   mingw64)
-    instdir=/cygdrive/c/ocamlmgw64
+    instdir='C:/ocamlmgw64'
     configure=nt
     cleanup=true
   ;;
   msvc)
-    instdir=/cygdrive/c/ocamlms
+    instdir='C:/ocamlms'
     configure=nt
     cleanup=true
   ;;
   msvc64)
-    instdir=/cygdrive/c/ocamlms64
+    instdir='C:/ocamlms64'
     configure=nt
     cleanup=true
   ;;