Disambiguate the provided to cmake path to be a src dir

cmake treats the provided path differently depending on whether it
contains a CMakeCache.txt or not. If it doesn't contain it -- it's
treated as a path to the source tree, if it does -- as a path to the
build tree. We want it to be treated as a source tree path, but if a
user has CMakeCache.txt in that directory, e.g. from a previous in-tree
build the user has done, cmake will treat it as a build tree instead,
which might lead to unexpected results (improperly configured build) or
an error, with the latter being more likely considering we are building
inside a container and the host paths specified in the user-generated
CMakeCache.txt likely don't exist in there.

other/docker/windows/build_toxcore.sh

@@ -70,7 +70,7 @@ build() {
     -DCMAKE_EXE_LINKER_FLAGS="$CMAKE_EXE_LINKER_FLAGS -fstack-protector" \
     -DCMAKE_SHARED_LINKER_FLAGS="$CMAKE_SHARED_LINKER_FLAGS" \
     $EXTRA_CMAKE_FLAGS \
-    ..
+    -S ..
   cmake --build . --target install -- -j"$(nproc)"
 
   if [ "$ENABLE_TEST" = "true" ]; then