Make the zlib dev package available in CentOS 8 and AmazonLinux2 images.

The intention of this change is to improve consistency between the various platform images. The libz headers and libraries are not a dependency of Swift at runtime on any OS, but are a common need for libraries such as `swift-nio-extras`, and are provided by the Ubuntu images. The resulting images were less than 60KB larger in local testing.

Author: gwynne

nightly-master/amazonlinux/2/Dockerfile

@@ -17,7 +17,8 @@ RUN yum -y update && yum -y install \
   libuuid \
   libxml2 \
   tar \
-  tzdata
+  tzdata \
+  zlib-devel
 
 # Everything up to here should cache nicely between Swift versions, assuming dev dependencies change little
 

nightly-master/centos/8/Dockerfile

@@ -16,7 +16,8 @@ RUN yum -y update && yum install --enablerepo=PowerTools -y \
   libstdc++-static \
   pkg-config \
   python2 \
-  sqlite
+  sqlite \
+  zlib-devel
 
 RUN ln -s /usr/bin/python2 /usr/bin/python