Update workflow to use GITHUB_WORKSPACE

Update workflow to use GITHUB_WORKSPACE

It was pointed out the GITHUB_WORKSPACE points to the container path of
the workspace, so we can use it instead of hardcoding the
__w/openssl/openssl path

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22726)