scripts: fail if hehlm is not present in PATH

To build OLM on minishift it is required to have the helm binary
available so adding a check to verify its existence.

Signed-off-by: Sébastien Han <seb@redhat.com>

scripts/build_local_shift.sh

@@ -4,6 +4,12 @@
 # This is used to start and build services for running e2e tests
 
 set -e
+
+if ! command -v helm &>/dev/null; then
+    echo "'helm' binary not present in your PATH, download it here: https://github.com/helm/helm/releases"
+    exit 1
+fi
+
 minishift start \
     || { echo 'Cannot start shift.'; exit 1; }