fix: stop flatpak DataChannel build from re-cloning over the sandboxed network

deps/DataChannel/DataChannel.cmake

@@ -1,6 +1,25 @@
 # libdatachannel is the native ICE/DTLS/SCTP implementation used by the
 # GUI WebRTC camera controller. Keep the source revision fixed: the signaling
 # protocol is evolving independently of this transport dependency.
+#
+# It vendors plog, usrsctp and libjuice as git submodules, which a plain
+# GitHub tag tarball does not include. The flatpak sandbox has no network
+# access during the build, so there the manifest itself clones the repo
+# (submodules and all) straight into this ExternalProject's default source
+# dir before the sandbox closes; with no download method given here and that
+# dir already non-empty, ExternalProject_Add silently uses it as-is instead
+# of trying its own (network) git step.
+if (FLATPAK)
+    set(_datachannel_source "")
+else()
+    set(_datachannel_source
+        GIT_REPOSITORY https://github.com/paullouisageneau/libdatachannel.git
+        GIT_TAG v0.24.5
+        GIT_SHALLOW ON
+        GIT_SUBMODULES_RECURSE ON
+    )
+endif()
+
 orcaslicer_add_cmake_project(DataChannel
     DEPENDS ${OPENSSL_PKG}
     CMAKE_ARGS
@@ -13,8 +32,5 @@ orcaslicer_add_cmake_project(DataChannel
         -DUSE_SYSTEM_USRSCTP=OFF
         -DOPENSSL_ROOT_DIR:PATH=${DESTDIR}
         -DOPENSSL_USE_STATIC_LIBS=ON
-    GIT_REPOSITORY https://github.com/paullouisageneau/libdatachannel.git
-    GIT_TAG v0.24.5
-    GIT_SHALLOW ON
-    GIT_SUBMODULES_RECURSE ON
+    ${_datachannel_source}
 )