The release tarball file doesn't contain submodules

[lcgogo]

Describe the bug
The release tarball file tar.gz doesn't contain submodules

Can use the followed suggestion to add the submodule folder.
AcademySoftwareFoundation/MaterialX#317 (comment)

To Reproduce

$ cd /tmp/
$ wget https://github.com/OffchainLabs/nitro/archive/refs/tags/v2.0.7.tar.gz
$ tar -zxf v2.0.7.tar.gz
cd nitro-2.0.7
$ git submodule update --init --recursive --force
$ git submodule
$ ls -l brotli
total 0
$ cat .gitmodules
[submodule "go-ethereum"]
	path = go-ethereum
	url = https://github.com/OffchainLabs/go-ethereum.git
[submodule "fastcache"]
	path = fastcache
	url = https://github.com/OffchainLabs/fastcache.git
[submodule "arbitrator/wasm-libraries/soft-float/SoftFloat"]
	path = arbitrator/wasm-libraries/soft-float/SoftFloat
	url = https://github.com/OffchainLabs/SoftFloat.git
[submodule "brotli"]
	path = brotli
	url = https://github.com/google/brotli.git
[submodule "blockscout"]
	path = blockscout
        url = https://github.com/OffchainLabs/blockscout.git
[submodule "arbitrator/wasm-testsuite/testsuite"]
	path = arbitrator/wasm-testsuite/testsuite
	url = https://github.com/WebAssembly/testsuite.git

[joshuacolvin0]

tarball is created by github, we have no plans to create custom tarball since the recommended way to get source is using git clone.