[0.20] lint: fix shellcheck URL in CI install
This is causing the tests to fail for backports i.e #19606. If you look in the Travis logs there, the output is:
You are downloading ShellCheck from an outdated URL!
Please update to the new URL:
https://github.com/koalaman/shellcheck/releases/download/v0.6.0/shellcheck-v0.6.0.linux.x86_64.tar.xz
For more information, see:
https://github.com/koalaman/shellcheck/issues/1871
PS: Sorry for breaking your build. The hosting costs were getting out of hand :(