Allow override of protobuf_FORCE_FETCH_DEPENDENCIES

[ngoquang2708]

flatpak-builder disallow internet access in build-time.

This allow us to build protobuf's dependencies (abseil in this case) with clang to avoid linking issue.

Ref: flathub/net.shadps4.shadPS4#77.

[squidbus]

@georgemoralis Is this something we needed to download at build time to begin with, feel like we could've just submodule'd this ourselves.

[georgemoralis]

not sure how that switch works ;/

[ngoquang2708]

protobuf_FORCE_FETCH_DEPENDENCIES force protobuf to fetch and build abseil by it self.
Currently we hardcoded protobuf_FORCE_FETCH_DEPENDENCIES to ON.
This PR allow us to override that.
@squidbus suggestion is good to me.

[ConnorG512]

Nix also dissalows internet access through building. I worked around it also by pulling the abseil source before the build and overwriting the source path when CMake calls its fetch content. If we can use a submodule for it then that step could probably be avoided completely.