refactor(guix-1.5.0): [not tested]split seed prep into improve step and move build script to pass2

steps-guix/guix-1.5.0/pass2.sh

@@ -8,10 +8,6 @@ src_prepare() {
 
     default
 
-    # Merge improve:guix-1.5.0 into build:guix-1.5.0 so manifest order cannot
-    # accidentally influence pass number selection.
-    "${SRCDIR}/improve/guix-1.5.0.sh"
-
     if [ ! -f /tmp/guix-bootstrap-seeds.env ]; then
         echo "Missing /tmp/guix-bootstrap-seeds.env" >&2
         false

steps-guix/manifest

@@ -69,6 +69,7 @@ build: mes-minimal-stripped-0.19-i686-linux
 build: mescc-tools-static-stripped-0.5.2-i686-linux
 # End of Guix Bootstrap seeds
 build: guix-hash-compat-1.5.0
+improve: guix-1.5.0
 build: guix-1.5.0
 build: loopback-bind-127001
 improve: services