Fix gcc install check

[timdawborn]

Currently, if you attempt to run setup-fortran.sh on a base Ubuntu image, if the version of gcc that that image is bundled with happens to be the version of gcc you want to install, but gcc hasn't actually been installed yet, this script will fail, as it incorrectly assumes that if the version of gcc in apt show matches, that gcc is installed. apt show only shows you what the packages are; it does not say what is installed. You can check what has been installed via apt list --installed.

Example:

$ docker run --rm -it ubuntu:24.04
root@e493e172c0d1:/# version=13
root@e493e172c0d1:/# apt-get update
...
Reading package lists...
root@e493e172c0d1:/# cur=$(apt show gcc | grep "Version" | cut -d':' -f3 | cut -d'-' -f1)
root@e493e172c0d1:/# maj=$(echo $cur | cut -d'.' -f1)
root@e493e172c0d1:/# if [ "$maj" == "$version" ]; then echo "GCC $version already installed"; fi
GCC 13 already installed
root@e493e172c0d1:/# update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-${version} 100
update-alternatives: error: alternative path /usr/bin/gcc-13 doesn't exist
root@e493e172c0d1:/# gcc
bash: gcc: command not found
root@e493e172c0d1:/# gcc-13
bash: gcc-13: command not found

This logic error doesn't present itself when running on GitHub Actions's ubuntu-latest image as system gcc is already installed.

This PR fixes the logic error in checking whether gcc has been installed.

[wpbonelli]

yikes. this one is on me.