CMake install doesn't honor install prefix

Nobody has claimed this yet.

CMake's install command line allows the user to set a custom prefix after configure / build. The install rules in mimalloc are not written to honor this and unconditionally installs to `/usr/local` (the default `CMAKE_INSTALL_PREFIX`).

```
$ git clone git@github.com:microsoft/mimalloc.git
$ cmake -S mimalloc -B mimalloc-build -DCMAKE_BUILD_TYPE=Release
$ cmake --build mimalloc-build
$ cmake --install mimalloc-build --prefix ./mimalloc-install
-- Install configuration: "Release"
-- Installing: /usr/local/lib/mimalloc-1.6/libmimalloc.so.1.6
-- Installing: /usr/local/lib/mimalloc-1.6/libmimalloc.so
-- Installing: /usr/local/lib/mimalloc-1.6/cmake/mimalloc.cmake
-- Installing: /usr/local/lib/mimalloc-1.6/cmake/mimalloc-release.cmake
-- Installing: /usr/local/lib/mimalloc-1.6/libmimalloc.a
-- Installing: /usr/local/lib/mimalloc-1.6/include/mimalloc.h
-- Installing: /usr/local/lib/mimalloc-1.6/include/mimalloc-override.h
-- Installing: /usr/local/lib/mimalloc-1.6/include/mimalloc-new-delete.h
-- Installing: /usr/local/lib/mimalloc-1.6/cmake/mimalloc-config.cmake
-- Installing: /usr/local/lib/mimalloc-1.6/cmake/mimalloc-config-version.cmake
-- Symbolic link: /usr/local/lib/libmimalloc.so -> mimalloc-1.6/libmimalloc.so.1.6
-- Installing: /usr/local/lib/mimalloc-1.6/mimalloc.o
```

This should have installed to `./mimalloc-install` instead. It is possible to work around this by setting `CMAKE_INSTALL_PREFIX` at configure time, but this should not be necessary.

Reproduce the issue with the listed cmake configure, build, and install commands, then inspect mimalloc's CMake install rules. Verify that `cmake --install mimalloc-build --prefix ./mimalloc-install` places the library, headers, CMake files, and object file under `./mimalloc-install` rather than `/usr/local`.

Written by the indexing model from the issue text.