HTML-installing targets remove the other one

This is actually a build configuration task, but I don't have a tool to
do that (no `./configure.sh` to run!).

To work around this, I'll just have each of the html-installing targets
install their own and remove the other. That way back-to-back builds
don't accidentally contaminate each other. The WASM and JS files are
identical, so they don't need this treatment.

Makefile

@@ -38,10 +38,12 @@ out/boids.js out/boids_bg.wasm &: target/$(CARGO_TARGET)/$(CARGO_PROFILE)/anothe
 # Copies the index page to the output
 out/index.html: www/index.html
 	cp -a $< $@
+	rm -f out/boids.html
 
 # Like `out/index.html`, but renames it for use in a larger site.
 out/boids.html: www/index.html
 	cp -a $< $@
+	rm -f out/index.html
 
 # Clean the web build, but not the Cargo cache. Cargo handles it's own caching
 # and I don't want to obliterate it all the time.