Don't force /usr to be merged until after we have built the sysroot

In some older suites, building packages will fail in a merged-/usr
chroot (#843073). Building packages in a merged-/usr chroot is also,
in general, not guaranteed to result in binaries that work correctly in
an unmerged-/usr environment, so we should probably prefer to build a
more traditional sysroot.
Signed-off-by: Simon McVittie <smcv@collabora.com>

flatdeb/debos-base.yaml

@@ -42,10 +42,10 @@ actions:
     {{ if $keyringpackage }}
     keyring-package: {{ $keyringpackage }}
     {{ end }}
-    {{ if eq $mergedusr "after_debootstrap" }}
-    merged-usr: false
+    {{ if eq $mergedusr "true" }}
+    merged-usr: true
     {{ else }}
-    merged-usr: {{ $mergedusr }}
+    merged-usr: false
     {{ end }}
     variant: minbase
     # TODO: flatdeb has historically used a configurable

flatdeb/debos-runtimes.yaml

@@ -70,6 +70,11 @@ actions:
   {{ end }}
 
   {{ if not $sdk }}
+  - action: run
+    label: usrmerge
+    chroot: false
+    command: 'echo; "$RECIPEDIR/usrmerge" "$ROOTDIR"'
+
   - action: run
     label: platformize
     chroot: false
@@ -172,11 +177,6 @@ actions:
     command: 'echo; dpkg --purge --force-remove-essential --force-depends dpkg'
   {{ end }}
 
-  - action: run
-    label: usrmerge
-    chroot: false
-    command: 'echo; "$RECIPEDIR/usrmerge" "$ROOTDIR"'
-
   # This adds the Flatpak metadata and the final apt configuration
   - action: overlay
     source: runtimes/{{ $runtime }}/overlay
@@ -238,6 +238,11 @@ actions:
       .
   {{ end }}
 
+  - action: run
+    label: usrmerge
+    chroot: false
+    command: 'echo; "$RECIPEDIR/usrmerge" "$ROOTDIR"'
+
   - action: run
     label: symlink-alternatives
     chroot: true