refresh certs in C# distribtest dockerfiles

tools/dockerfile/distribtest/csharp_centos7_x64/Dockerfile

@@ -22,3 +22,6 @@ RUN yum install -y mono-devel
 RUN yum install -y nuget
 
 RUN yum install -y unzip
+
+# Make sure the mono certificate store is up-to-date to prevent issues with nuget restore
+RUN curl https://curl.haxx.se/ca/cacert.pem > ~/cacert.pem && cert-sync ~/cacert.pem && rm -f ~/cacert.pem

tools/dockerfile/distribtest/csharp_jessie_x64/Dockerfile

@@ -25,3 +25,7 @@ RUN apt-get update && apt-get install -y \
     && apt-get clean
 
 RUN apt-get update && apt-get install -y unzip && apt-get clean
+
+# Make sure the mono certificate store is up-to-date to prevent issues with nuget restore
+RUN apt-get update && apt-get install -y curl && apt-get clean
+RUN curl https://curl.haxx.se/ca/cacert.pem > ~/cacert.pem && cert-sync ~/cacert.pem && rm -f ~/cacert.pem

tools/dockerfile/distribtest/csharp_jessie_x86/Dockerfile

@@ -25,3 +25,7 @@ RUN apt-get update && apt-get install -y \
     && apt-get clean
 
 RUN apt-get update && apt-get install -y unzip && apt-get clean
+
+# Make sure the mono certificate store is up-to-date to prevent issues with nuget restore
+RUN apt-get update && apt-get install -y curl && apt-get clean
+RUN curl https://curl.haxx.se/ca/cacert.pem > ~/cacert.pem && cert-sync ~/cacert.pem && rm -f ~/cacert.pem

tools/dockerfile/distribtest/csharp_ubuntu1404_x64/Dockerfile

@@ -38,3 +38,7 @@ RUN mkdir warmup \
     && dotnet new \
     && cd .. \
     && rm -rf warmup
+
+# Make sure the mono certificate store is up-to-date to prevent issues with nuget restore
+RUN apt-get update && apt-get install -y curl && apt-get clean
+RUN curl https://curl.haxx.se/ca/cacert.pem > ~/cacert.pem && cert-sync ~/cacert.pem && rm -f ~/cacert.pem

tools/dockerfile/distribtest/csharp_ubuntu1604_x64/Dockerfile

@@ -25,3 +25,7 @@ RUN apt-get update && apt-get install -y \
     && apt-get clean
 
 RUN apt-get update && apt-get install -y unzip && apt-get clean
+
+# Make sure the mono certificate store is up-to-date to prevent issues with nuget restore
+RUN apt-get update && apt-get install -y curl && apt-get clean
+RUN curl https://curl.haxx.se/ca/cacert.pem > ~/cacert.pem && cert-sync ~/cacert.pem && rm -f ~/cacert.pem