Update Ubuntu images to 20.04

ci/docker/aarch64-linux-android/Dockerfile

ci/docker/aarch64-unknown-linux-gnu/Dockerfile

ci/docker/aarch64-unknown-linux-musl/Dockerfile

ci/docker/arm-linux-androideabi/Dockerfile

ci/docker/arm-unknown-linux-gnueabihf/Dockerfile

ci/docker/arm-unknown-linux-musleabihf/Dockerfile

ci/docker/asmjs-unknown-emscripten/Dockerfile

ci/docker/i686-linux-android/Dockerfile

ci/docker/i686-unknown-linux-gnu/Dockerfile

ci/docker/i686-unknown-linux-musl/Dockerfile

ci/docker/mips-unknown-linux-musl/Dockerfile

ci/docker/mips64-unknown-linux-muslabi64/Dockerfile

ci/docker/mips64el-unknown-linux-muslabi64/Dockerfile

ci/docker/mipsel-unknown-linux-musl/Dockerfile

ci/docker/powerpc-unknown-linux-gnu/Dockerfile

ci/docker/powerpc64-unknown-linux-gnu/Dockerfile

ci/docker/powerpc64le-unknown-linux-gnu/Dockerfile

ci/docker/s390x-unknown-linux-gnu/Dockerfile

ci/docker/sparc64-unknown-linux-gnu/Dockerfile

ci/docker/wasm32-unknown-emscripten/Dockerfile