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/build/fietsje/deps_monogon.go b/build/fietsje/deps_monogon.go
index 2869722..021ce48 100644
--- a/build/fietsje/deps_monogon.go
+++ b/build/fietsje/deps_monogon.go
@@ -69,6 +69,8 @@
 		"github.com/ulikunitz/xz",
 	)
 
+	p.collect("github.com/pierrec/lz4/v4", "v4.1.12")
+
 	// Used by //metropolis/build/genosrelease
 	p.collect("github.com/joho/godotenv", "v1.3.0")
 
@@ -137,6 +139,9 @@
 		"github.com/pseudomuto/protokit",
 	)
 
+	// Used by //metropolis/node/build/mkcpio
+	p.collect("github.com/cavaliergopher/cpio", "b4d3577d12deb76a6d52f184acd341f875574785")
+
 	// First generate the repositories starlark rule into memory. This is because
 	// rendering will lock all unlocked dependencies, which might take a while. If a
 	// use were to interrupt it now, they would end up with an incomplete