m/n/build: implement new fsspec infrastructure

This makes the node_initramfs and erofs_image use the new common fsspec
infrastructure. It also adds the fsspecs attribute to both which can
later be used to add arbitrary fsspecs.

Change-Id: I384e04712c0a70f82c5c975911cbb1d0d5e6cabc
Reviewed-on: https://review.monogon.dev/c/monogon/+/530
Reviewed-by: Sergiusz Bazanski <serge@monogon.tech>
diff --git a/metropolis/test/launch/cluster/cluster.go b/metropolis/test/launch/cluster/cluster.go
index 9b32f97..bd3a76a 100644
--- a/metropolis/test/launch/cluster/cluster.go
+++ b/metropolis/test/launch/cluster/cluster.go
@@ -380,7 +380,7 @@
 	go func() {
 		if err := launch.RunMicroVM(ctxT, &launch.MicroVMOptions{
 			KernelPath:             "metropolis/test/ktest/vmlinux",
-			InitramfsPath:          "metropolis/test/nanoswitch/initramfs.lz4",
+			InitramfsPath:          "metropolis/test/nanoswitch/initramfs.cpio.lz4",
 			ExtraNetworkInterfaces: switchPorts,
 			PortMap:                portMap,
 		}); err != nil {