Add github action to build and test with mmap emulation

.github/workflows/nightly.yml

@@ -96,6 +96,17 @@ jobs:
             ASAN_SYMBOLIZER_PATH: /usr/bin/llvm-symbolizer-10
             TSAN_OPTIONS: suppressions=/home/libgit2/source/script/thread-sanitizer.supp second_deadlock_stack=1
           os: ubuntu-latest
+        - # Focal, Clang 10, mmap emulation (NO_MMAP)
+          container:
+            name: focal
+          env:
+            CC: clang-10
+            CFLAGS: -DNO_MMAP
+            CMAKE_OPTIONS: -DCMAKE_PREFIX_PATH=/usr/local
+            CMAKE_GENERATOR: Ninja
+            SKIP_SSH_TESTS: true
+            SKIP_NEGOTIATE_TESTS: true
+          os: ubuntu-latest
         - # macOS
           os: macos-10.15
           env:
@@ -114,6 +125,15 @@ jobs:
             CMAKE_OPTIONS: -A x64 -DWIN32_LEAKCHECK=ON -DDEPRECATE_HARD=ON
             SKIP_SSH_TESTS: true
             SKIP_NEGOTIATE_TESTS: true
+        - # Windows amd64 Visual Studio (NO_MMAP)
+          os: windows-2019
+          env:
+            ARCH: amd64
+            CMAKE_GENERATOR: Visual Studio 16 2019
+            CFLAGS: -DNO_MMAP
+            CMAKE_OPTIONS: -A x64 -DDEPRECATE_HARD=ON
+            SKIP_SSH_TESTS: true
+            SKIP_NEGOTIATE_TESTS: true
         - # Windows x86 Visual Studio
           os: windows-2019
           env: