CI: Fix a minor scripting error

Author: bonachea

.github/workflows/build.yml

@@ -157,7 +157,7 @@ jobs:
         sudo apt update
         #sudo apt list -a 'gfortran-*'
         sudo apt install -y build-essential
-        if [[ ${COMPILER_VERSION} < 15 ]] ; then \
+        if (( ${COMPILER_VERSION} < 15 )) ; then \
           sudo apt install -y gfortran-${COMPILER_VERSION} ; \
         else \
           curl -L https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh -o install-homebrew.sh ; \