gitlab-ci: Move scons build/test commands to a separate shell script

Preparatory, no functional change intended.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

.gitlab-ci/scons-build.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+
+set -e
+set -o xtrace
+
+if test -n "$LLVM_VERSION"; then
+    export LLVM_CONFIG="llvm-config-${LLVM_VERSION}"
+fi
+
+rm -rf build
+scons $SCONS_TARGET
+eval $SCONS_CHECK_COMMAND