Fix: Folder deps need to be order-only deps

I did the thing againnnnn. The Makefile thinks the sound assets are
constantly out-of-date because the folder that contains them technically
changes any time a file is added or removed from the folder.

1. "out/assets" is created, it is up-to-date
2. "out/assets/example.ogg" is created, it is up-to-date
3. Parent folder "out/assets" has it's mtime updated because it's
   contents have changed. It is newer than it was at step 1, and newer
   than the .ogg file in step 2.
4. run `make` again
5. "out/assets" is up-to-date, nothing changes
6. "out/assets/example.ogg" is OLDER than one of it's dependencies
   ("out/assets/"), and is rebuilt.
7. "out/assets" got new contents, so it's mtime was updated again.

The cycle isn't infinite, but it will always try to rebuild the sound
files. The fix is to consider the containing folder to only be an
ordering dependency rather than a substantive dependency. The former
only needs the dependency to be made first, where the latter considers
the dependency to be part of the target file. The containing folder is
not part of the sound files, so "rebuilding" the sound files when the
folder changes is complete nonsense.

Makefile

@@ -40,10 +40,10 @@ target/$(CARGO_TARGET)/$(CARGO_PROFILE)/asteroids.wasm: $(SRCS) Cargo.lock Cargo
 out:
 	mkdir $@
 
-out/assets: out
+out/assets: | out
 	mkdir $@
 
-out/assets/%.ogg: out/assets assets/%.ogg
+out/assets/%.ogg: assets/%.ogg | out/assets
 	cp -ar assets/$*.ogg $@
 
 # Both the JS and WASM files are generated by the wasm-bindgen call, so both