hst: fix ephemeral overlay order

This is quite counterintuitive otherwise.

Signed-off-by: Ophestra <cat@gensokyo.uk>

hst/fsoverlay.go

@@ -82,9 +82,9 @@ func (o *FSOverlay) Apply(z *ApplyState) {
 			z.Overlay(o.Target, o.Upper, o.Work, o.Lower...)
 		} else {
 			z.OverlayEphemeral(o.Target, slices.Concat(
+				[]*check.Absolute{o.Upper},
 				o.Lower,
-				[]*check.Absolute{o.Upper})...,
-			)
+			)...)
 		}
 	} else {
 		z.OverlayReadonly(o.Target, o.Lower...)

hst/fsoverlay_test.go

@@ -70,7 +70,7 @@ func TestFSOverlay(t *testing.T) {
 			Upper:  m("/tmp/upper"),
 		}, true, container.Ops{&container.MountOverlayOp{
 			Target: m("/"),
-			Lower:  ms("/tmp/.src0", "/tmp/.src1", "/tmp/upper"),
+			Lower:  ms("/tmp/upper", "/tmp/.src0", "/tmp/.src1"),
 			Upper:  fhs.AbsRoot,
 		}}, m("/"), ms("/tmp/upper", "/tmp/.src0", "/tmp/.src1"),
 			"e*/:/tmp/upper:/tmp/.src0:/tmp/.src1"},