Update the nightly toolchains to not use yum update

Author: shahmishal

nightly-master/amazonlinux/2/Dockerfile

@@ -2,7 +2,7 @@ FROM amazonlinux:2
 LABEL maintainer="Swift Infrastructure <[email protected]>"
 LABEL description="Docker Container for the Swift programming language"
 
-RUN yum -y update && yum -y install \
+RUN yum -y install \
   binutils \
   gcc \
   git \

nightly-master/centos/7/Dockerfile

@@ -2,7 +2,7 @@ FROM centos:7
 LABEL maintainer="Swift Infrastructure <[email protected]>"
 LABEL description="Docker Container for the Swift programming language"
 
-RUN yum -y update && yum install shadow-utils.x86_64 -y \
+RUN yum install shadow-utils.x86_64 -y \
   binutils \
   gcc \
   git \

nightly-master/centos/8/Dockerfile

@@ -4,7 +4,7 @@ LABEL description="Docker Container for the Swift programming language"
 
 RUN yum install -y https://dl.fedoraproject.org/pub/epel/epel-release-latest-8.noarch.rpm
 
-RUN yum -y update && yum install --enablerepo=PowerTools -y \
+RUN yum install --enablerepo=PowerTools -y \
   binutils \
   gcc \
   git \