fix: use ubuntu20 to build release image

[1xstj]

Summary of changes
Changes introduced in this pull request:

• Use older ubuntu version (v20) to build the release image, this will avoid the following errors on older systems

./tangle-testnet-linux-amd64: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.32' not found (required by ./tangle-testnet-linux-amd64)
./tangle-testnet-linux-amd64: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by ./tangle-testnet-linux-amd64)
./tangle-testnet-linux-amd64: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.33' not found (required by ./tangle-testnet-linux-amd64)

For systems with older than ubuntu20, will have to build from source since github runners does not support those.