Evaluate a build product (e.g., image)

[crabdancing]

There currently seems to be no general-purpose way of using a Colmena hive expression to generate arbitrary build products (e.g., boot images).

Say we have a node called mynode.

If I do colmena repl, and then in the REPL, run: :b nodes.mynode.config.system.build.sdImage, then it will correctly build the microSD image. Same for ISOs, etc. It would be nice to add a command to build an arbitrary target in the colmena expression, e.g., colmena build-product nodes.mynode.config.system.build.sdImage. This could then be simplified by allowing the user to have a buildProducts attribute in the meta part of the Colmena hive schema, which could be as follows:

. . .
# assuming `rec` on global lambda
meta = {
  buildProducts = {
    mynode.sdImage = nodes.mynode.config.system.build.sdImage;
  };
};
. . .

Then you could do:

colmena build-product mynode.sdImage

To build the sdImage of mynode.