ci: install gfortran when the CUTEst optimization benchmarks are run

Author: thazhemadam

.buildkite/build_benchmark.sh

@@ -18,6 +18,10 @@ fi
 echo "--- :julia: Instantiate"
 julia --project=. -e 'using Pkg; Pkg.instantiate(); Pkg.build()'
 
+if [[ "${1}" == *OptimizationCUTEst* ]]; then
+	apt update && apt install -y gfortran
+fi
+
 if [[ "${1}" == *BayesianInference* ]]; then
 	export CMDSTAN_HOME="$(pwd)/cmdstan-2.29.2/"
 	curl -LO https://github.com/stan-dev/cmdstan/releases/download/v2.29.2/cmdstan-2.29.2.tar.gz