.github: Add shared lib builds (#3975)

So far, no workflows would attempt to build the shared version of the
iwasm library (namely, vmlib).

Note that, as opposed to GC_EH_BUILD_OPTIONS and DEFAULT_BUILD_OPTIONS,
the actual default options defined by the build system are assumed, for
the sake of simplicity and avoiding repeated code.

.github/workflows/nightly_run.yml

@@ -142,6 +142,7 @@ jobs:
             "-DWAMR_DISABLE_HW_BOUND_CHECK=1",
             "-DWAMR_BUILD_MEMORY64=1",
             "-DWAMR_BUILD_MULTI_MEMORY=1",
+            "-DWAMR_BUILD_SHARED=1",
           ]
         os: [ubuntu-20.04]
         platform: [android, linux]