scan_for_configure can cause build_patterns to switch between builds

Due to the way os.walk is implemented using readdir, the order that files are inspected to determine the build pattern for a project is potentially arbitrary. This can result in a project with multiple files that can be used for build pattern detection to switch between build systems upon autospec being rerun on the project.

I ran into this issue yesterday with the Clear Linux gnome-calculator package. See the commit at https://github.com/clearlinux-pkgs/gnome-calculator/commit/cde0c41a85, where I needed to set the meson build pattern manually, whereas before, that pattern was autodetected.