ci: don't use --insecure

mbedTLS has fixed their certificate.  🎉

Author: ethomson

azure-pipelines/docker/bionic

@@ -25,7 +25,7 @@ RUN apt-get update && \
 
 FROM apt AS mbedtls
 RUN cd /tmp && \
-    curl --insecure --location --silent --show-error https://tls.mbed.org/download/mbedtls-2.16.2-apache.tgz | \
+    curl --location --silent --show-error https://tls.mbed.org/download/mbedtls-2.16.2-apache.tgz | \
     tar -xz && \
     cd mbedtls-2.16.2 && \
     scripts/config.pl set MBEDTLS_MD4_C 1 && \

azure-pipelines/docker/xenial

@@ -29,7 +29,7 @@ RUN apt-get update && \
 
 FROM apt AS mbedtls
 RUN cd /tmp && \
-    curl --insecure --location --silent --show-error https://tls.mbed.org/download/mbedtls-2.16.2-apache.tgz | \
+    curl --location --silent --show-error https://tls.mbed.org/download/mbedtls-2.16.2-apache.tgz | \
     tar -xz && \
     cd mbedtls-2.16.2 && \
     scripts/config.pl set MBEDTLS_MD4_C 1 && \