bootstrap_sdk: override seed path after building stage1

When performing a full bootstrap (stage1-4), the stage1 results are currently
discarded because of the logic in catalyst_build: the first build stage uses
the "seed" and every following stage uses the previous stages results *but*
stage1 is built before catalyst_build. So from the point of view of
catalyst_build, stage2 is the first one and uses the seed tarball.

To make sure stage1 results are used if it was built, set the SEED variable to
the latest stage1 location.

Signed-off-by: Jeremi Piotrowski <jpiotrowski@microsoft.com>

bootstrap_sdk

@@ -198,6 +198,7 @@ build_stage1() {
 if [[ "$STAGES" =~ stage1 ]]; then
     build_stage1
     STAGES="${STAGES/stage1/}"
+    SEED="${TYPE}/stage1-${ARCH}-latest"
 fi
 
 catalyst_build