build: Use /etc/os-release rather than hardcoding distro version

So we have one fewer random place to change when bumping Fedora
versions.  Prep for rebasing on F31.

build.sh

@@ -22,13 +22,13 @@ fi
 set -x
 srcdir=$(pwd)
 
-release="30"
-
 configure_yum_repos() {
+    local version_id
+    version_id=$(. /etc/os-release && echo ${VERSION_ID})
     # Add continuous tag for latest build tools and mark as required so we
     # can depend on those latest tools being available in all container
     # builds.
-    echo -e "[f$release-coreos-continuous]\nenabled=1\nmetadata_expire=1m\nbaseurl=https://kojipkgs.fedoraproject.org/repos-dist/f$release-coreos-continuous/latest/\$basearch/\ngpgcheck=0\nskip_if_unavailable=False\n" > /etc/yum.repos.d/coreos.repo
+    echo -e "[f${version_id}-coreos-continuous]\nenabled=1\nmetadata_expire=1m\nbaseurl=https://kojipkgs.fedoraproject.org/repos-dist/f${version_id}-coreos-continuous/latest/\$basearch/\ngpgcheck=0\nskip_if_unavailable=False\n" > /etc/yum.repos.d/coreos.repo
 
 }