Update cli for new platform host build process about roc

[lukewilliamboswell]

Also see this zulip chat

@rtfeldman said:

• we always build applications from a precompiled host, so no more 🔨 Rebuilding platform... message
• this means that the individual platform needs to have a script which does the relevant parts of "rebuilding platform" for its host
• the goal of the platform build script should be outputting a Roc precompiled host file

@brian-carroll siad:

OK so we should start recommending that to people who are having build problems.

1. Use ROC_PRINT_BUILD_COMMANDS=1 roc build my-program.roc to see the commands that Roc is using to build your host language code.
2. Extract those commands into your favourite build system, like a build.zig or a build.rs or a makefile.
3. From then on, use roc build my-program.roc --precompiled-host=<path> and change the commands as you see fit.

ROC_PRINT_BUILD_COMMANDS=1 will only work with a development build of the Roc compiler.

[Anton-4]

Perhaps we should create a new issue that combines #6307 and this one?