github-pages workflow deploy fail

[dieterplex]

From recent change, the deploy is fail due to:

Run peaceiris/actions-mdbook@v1
mdbook version: 0.4.21
Operating System: unknown-linux-gnu
toolURL: https://github.com/rust-lang/mdBook/releases/download/v0.4.21/mdbook-v0.4.21-x86_64-unknown-linux-gnu.tar.gz
/bin/tar xz --warning=no-unknown-keyword -C /home/runner/tmp -f /home/runner/work/_temp/bc8b8fa7-2411-4a7b-b503-cecaabd29ffd
/home/runner/toolbin/mdbook --version
/home/runner/toolbin/mdbook: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.29' not found (required by /home/runner/toolbin/mdbook)
/home/runner/toolbin/mdbook: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found (required by /home/runner/toolbin/mdbook)

Run mdbook build
mdbook: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.29' not found (required by mdbook)
mdbook: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found (required by mdbook)
Error: Process completed with exit code 1.

It has been reported to actions-mdbook: https://github.com/peaceiris/actions-mdbook/issues/464

Another issue is there is no package: linuxbrew-wrapper in runner-image ubuntu 20.04/2.04 (see this action run of #151 ). #50 should address this issue.

[dieterplex]

151 only fix the initial issue. Although Homebrew is installed both on 18.04/20.04 runner, there is no linuxbrew-wrapper could be found in 20.04.