Fixes: #813

Otherwise such errors like https://open.greenhost.net/openappstack/openappstack/-/jobs/89464
are not catched:

    $ sh ./install/install-openappstack.sh
    ✚ generating manifests
     manifests build completed
    ► installing components in flux-system namespace
    ✗ install failed: exec: "kubectl": executable file not found in $PATH
    ✚ generating GitRepository source
    ► applying GitRepository source
     GitRepository source created
    ◎ waiting for GitRepository source reconciliation
     GitRepository source reconciliation completed
     fetched revision: master/fe6c3302
    ✚ generating Kustomization
    ► applying Kustomization
     Kustomization created
    ◎ waiting for Kustomization reconciliation
     Kustomization openappstack is ready
     applied revision master/fe6c3302