derivation: parse nix derivation show

This time it's JSON, and actual intended behaviour.

cmd/nixbuild/main.go

@@ -42,7 +42,20 @@ func main() {
 		Flag(&flagVerbose, "v", command.BoolFlag(false), "Connect nix stderr").
 		Flag(&flagJSON, "json", command.BoolFlag(false), "Serialise output in JSON when applicable")
 
-	c.Command("build", "Build a list of installables", func(args []string) error {
+	c.Command("show", command.UsageInternal, func(args []string) error {
+		if len(args) < 1 {
+			return commandHandlerError("usage requires at least 1 argument")
+		}
+
+		if drv, err := nixbuild.DerivationShow(ctx, slices.Values(args)); err != nil {
+			return commandHandlerError(fmt.Sprintf("cannot show: %v", err))
+		} else {
+			log.Printf("got %d derivations:\n%#v", len(drv), drv)
+			return nil
+		}
+	})
+
+	c.Command("build", command.UsageInternal, func(args []string) error {
 		if len(args) < 1 {
 			return commandHandlerError("build requires at least 1 argument")
 		}