Fix CentOS7 setup

C7 is EOL, has been moved to vault.centos.org.

.github/workflows/selinux.yml

@@ -15,6 +15,10 @@ jobs:
     steps:
       - name: Prepare
         run: |
+          sed -i \
+              -e 's,^#baseurl=http://mirror.centos.org/centos/,baseurl=https://vault.centos.org/centos/,g' \
+              -e 's,^mirrorlist=http://mirrorlist.centos.org/,#&1,' \
+              /etc/yum.repos.d/*.repo
           yum install -y selinux-policy-devel
       - uses: actions/checkout@v2
       - name: Build