osbuild: set the cache-max-size to 20 G when checkpoints are defined

Adding --checkpoints to the osbuild call has no effect right now unless
a previous run of osbuild set the max size already.
Since we support passing checkpoint args to the osbuild call, let's also
set the cache size to a baseline default value to have those checkpoints
actually checkpoint.

pkg/osbuild/osbuild-exec.go

@@ -8,6 +8,8 @@ import (
 	"os"
 	"os/exec"
 	"strings"
+
+	"github.com/osbuild/images/pkg/datasizes"
 )
 
 // Run an instance of osbuild, returning a parsed osbuild.Result.
@@ -34,6 +36,11 @@ func RunOSBuild(manifest []byte, store, outputDirectory string, exports, checkpo
 		cmd.Args = append(cmd.Args, "--checkpoint", checkpoint)
 	}
 
+	if len(checkpoints) > 0 {
+		// set the cache-max-size to a reasonable size that the checkpoints actually get stored
+		cmd.Args = append(cmd.Args, "--cache-max-size", fmt.Sprint(20*datasizes.GiB))
+	}
+
 	if result {
 		cmd.Args = append(cmd.Args, "--json")
 		cmd.Stdout = &stdoutBuffer