Switch from CentOS to Alma Linux (#998)

Looks like the old CentOS mirrors are being decommissioned.

Author: alexcrichton

ci/docker/x86_64-linux/Dockerfile

@@ -1,5 +1,5 @@
-FROM centos:7
+FROM almalinux:8
 
-RUN yum install -y git gcc
+RUN dnf install -y git gcc
 
 ENV PATH=$PATH:/rust/bin