Pre-built Rust compiler fails on Ubuntu 20.04

[aleasims]

Our pre-built version of rustc has no compatible version of glibc on Ubuntu 20.04. If you run rustc on Ubuntu 20.04 now, you will most likely get an error:

~/.rustup/toolchains/zkllvm/bin/rustc: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by ~/.rustup/toolchains/zkllvm/bin/rustc)
~/.rustup/toolchains/zkllvm/bin/rustc: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.33' not found (required by ~/.rustup/toolchains/zkllvm/lib/librustc_driver-0619e71a169efef1.so)
~/.rustup/toolchains/zkllvm/bin/rustc: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by ~/.rustup/toolchains/zkllvm/lib/librustc_driver-0619e71a169efef1.so)
~/.rustup/toolchains/zkllvm/bin/rustc: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.32' not found (required by ~/.rustup/toolchains/zkllvm/lib/librustc_driver-0619e71a169efef1.so)
~/.rustup/toolchains/zkllvm/bin/rustc: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.33' not found (required by ~/.rustup/toolchains/zkllvm/lib/libstd-febc5b336d467395.so)
~/.rustup/toolchains/zkllvm/bin/rustc: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.32' not found (required by ~/.rustup/toolchains/zkllvm/lib/libstd-febc5b336d467395.so)
~/.rustup/toolchains/zkllvm/bin/rustc: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.34' not found (required by ~/.rustup/toolchains/zkllvm/lib/libstd-febc5b336d467395.so)