Build -gnu targets on ubuntu-20.04

[coolreader18]

This happens to be similar to cargo-bins/cargo-binstall#607; essentially, the ubuntu-latest workflow runner now points to ubuntu 22.04, which has a newer glibc version than e.g. Debian stable, so when you try to run the binaries built on it on Debian stable, you get errors like:

/usr/local/cargo/bin/cargo-watch: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.33' not found (required by /usr/local/cargo/bin/cargo-watch)
/usr/local/cargo/bin/cargo-watch: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.32' not found (required by /usr/local/cargo/bin/cargo-watch)
/usr/local/cargo/bin/cargo-watch: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by /usr/local/cargo/bin/cargo-watch)

This fixes the issue by explicitly specifying 20.04, which has a lower version of glibc.

[passcod]

This is in 8.2.2