build: prebuild binary_cache and assetstorage canister asset archives

[ghost]

It turns out that it takes ~99 seconds to build binary_cache.tgz from the contents of the directory at $DFX_ASSETS.

This change builds both binary_cache.tgz and assetstorage_canister.tgzonce per nix-build, rather than once per cargo build.

The remaining .tgz archives are still built at cargo build time, but they only take a handful of milliseconds to build.

Please note that the trick of pointing $DFX_ASSETS at an empty directory (for cargo check) will no longer work, but it is also no longer necessary.

These were the times observed to build these files prior to this change:

binary_cache elapsed: 99.150417878s
assetstorage_canister elapsed: 466.368594ms
language_bindings elapsed: 1.073618ms
new_project_files elapsed: 4.211441ms
new_project_node_files elapsed: 2.722148ms
Elapsed: 99.636307058s

After: (I didn't bother to break it down by file)

Elapsed: 38.458861ms

This also removes fn add_optional_assets because we no longer build dfx in two stages.