ci: run a threadsafe build in nightly

.github/workflows/nightly.yml

@@ -51,6 +51,14 @@ jobs:
             CMAKE_OPTIONS: -DUSE_HTTPS=mbedTLS -DUSE_SHA1=HTTPS -DREGEX_BACKEND=pcre -DDEPRECATE_HARD=ON -DUSE_LEAK_CHECKER=valgrind -DUSE_GSSAPI=ON
             CMAKE_GENERATOR: Ninja
           os: ubuntu-latest
+        - # Xenial, GCC, thread-free
+          container:
+            name: xenial
+          env:
+            CC: gcc
+            CMAKE_OPTIONS: -DTHREADSAFE=OFF -DDEPRECATE_HARD=ON -DUSE_LEAK_CHECKER=valgrind -DUSE_GSSAPI=ON
+            CMAKE_GENERATOR: Ninja
+          os: ubuntu-latest
         - # Focal, Clang 10, mbedTLS, MemorySanitizer
           container:
             name: focal