As it is available for Debian, then you can install it manually on Ubuntu 20.04 LTS which is based on bullseye.
Use commands below:
cd ~/Downloads
wget http://ftp.debian.org/debian/pool/main/d/dh-cargo/dh-cargo_24_all.deb
wget http://ftp.debian.org/debian/pool/main/r/rust-debcargo/debcargo_2.4.4-1_amd64.deb
wget http://ftp.debian.org/debian/pool/main/libg/libgit2/libgit2-1.1_1.1.0+dfsg.1-4_amd64.deb
sudo apt-get install ./libgit2-1.1_1.1.0+dfsg.1-4_amd64.deb ./dh-cargo_24_all.deb ./debcargo_2.4.4-1_amd64.deb
and then start using debcargo.
If you prefer to use Ubuntu repositories - use package from -proposed pocket:
cd ~/Downloads
wget http://archive.ubuntu.com/ubuntu/pool/universe/r/rust-debcargo/debcargo_2.4.2-1_amd64.deb
sudo apt-get install ./debcargo_2.4.2-1_amd64.deb