CI: Install make on Cygwin

Test '230 external project' uses make, but is too dumb to use the
platform conventions for shared library names in installed files
expected by shared_lib, so special case that.

.github/workflows/cygwin.yml

@@ -69,6 +69,7 @@ jobs:
             libgtk3-devel
             libxml2-devel
             libxslt-devel
+            make
             ninja
             python2-devel
             python3-devel

test cases/common/230 external project/test.json

@@ -1,6 +1,7 @@
 {
   "installed": [
-    { "type": "shared_lib",  "file": "usr/lib/foo" },
+    { "type": "shared_lib",  "file": "usr/lib/foo", "platform": "!cygwin" },
+    { "type": "file",  "file": "usr/lib/libfoo.dll", "platform": "cygwin" },
     { "type": "file",  "file": "usr/include/libfoo.h" },
     { "type": "file", "file": "usr/lib/pkgconfig/somelib.pc" }
   ]