tools/ci-build: honor instdir on non-configure builds
tools/ci-build: honor instdir on non-configure builds

The instdir environment variable is used by ci-build to define where
make install should install the compiler. Before this commit, a value was
assigned to instdir for the Windows builds that do not use configure,
but then this value was not taken into account, making the assignments useless
for those builds that do not use configure.

This commit fixes this by using instdir to override the value of PREFIX
in config/Makefile.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml