ci: Skip running compressed-size builds twice

[rschristian]

Because we have a prepare script that builds the app, our compressed size action was building the apps once post-install, then again as our build script, resulting in 2 wasted builds per CI run.

You can easily see Microbundle's output here in both the 'Install Dependencies' and 'Build using npm' steps.

[coveralls]

coverage: 99.609%. remained the same
when pulling d12d306 on ci/compressed-size
into bfdd189 on main.