build-container: use PREBUILT_BASE_IMAGE verbatim

When PREBUILT_BASE_IMAGE is provided by the pipeline it already carries
its OS tag (e.g. ":alma8"), so appending ":${VESPA_BUILDOS_LABEL}"
produced an invalid double-tagged reference like "repo:alma8:alma8".
Use the value as-is instead.

Author: arnej27959

.buildkite/build-container.sh

@@ -59,7 +59,7 @@ select_dockerfile() {
 
 target_option=""
 if [ "${PREBUILT_BASE_IMAGE:-}" != "" ]; then
-    VESPA_BASE_IMAGE="${PREBUILT_BASE_IMAGE}:${VESPA_BUILDOS_LABEL}"
+    VESPA_BASE_IMAGE=${PREBUILT_BASE_IMAGE}
     SYSTEM_TEST_BASE_IMAGE=${VESPA_BASE_IMAGE}
     target_option="--target vespa"
 fi