ci: update Ubuntu 16.04 test to 20.04 with MSRV

Update the Ubuntu 16.04 test to Ubuntu 20.04 and make it clear
that it is also testing the minimum supported Rust version for 6.0.

Note: Ubuntu 16.04 is now EOL.

.github/workflows/builds.yml

@@ -27,6 +27,8 @@ env:
   # could cause some steps to fail.
   RUST_VERSION_KNOWN: "1.49.0"
 
+  # The minimum version of Rust supported.
+  RUST_VERSION_MIN: 1.34.2
 jobs:
 
   prepare-deps:
@@ -939,12 +941,12 @@ jobs:
       - run: AFL_HARDEN=1 ac_cv_func_realloc_0_nonnull=yes ac_cv_func_malloc_0_nonnull=yes CFLAGS="-fsanitize=address -fno-omit-frame-pointer" CXXFLAGS=$CFLAGS CC=afl-clang-fast CXX=afl-clang-fast++ ./configure --enable-fuzztargets --disable-shared
       - run: AFL_HARDEN=1 make -j2
 
-  # An Ubuntu 16.04 build using the tarball generated in the CentOS 8
-  # build above.
-  ubuntu-16-04:
-    name: Ubuntu 16.04
+  # An Ubuntu 20.04 build using the tarball generated in the CentOS 8
+  # build above also testing the minimum supported Rust version.
+  ubuntu-20-04-msrv:
+    name: Ubuntu 20.04 (MSRV)
     runs-on: ubuntu-latest
-    container: ubuntu:16.04
+    container: ubuntu:20.04
     needs: centos-8
     steps:
       - name: Install dependencies
@@ -971,11 +973,12 @@ jobs:
                 libyaml-0-2 \
                 libyaml-dev \
                 make \
+                python3-distutils \
                 python3-yaml \
                 zlib1g \
                 zlib1g-dev
       - name: Install Rust
-        run: curl https://sh.rustup.rs -sSf | sh -s -- --default-toolchain 1.34.2 -y
+        run: curl https://sh.rustup.rs -sSf | sh -s -- --default-toolchain ${RUST_VERSION_MIN} -y
       - run: echo "$HOME/.cargo/bin" >> $GITHUB_PATH
       - name: Download suricata.tar.gz
         uses: actions/download-artifact@v2