Fix Antithesis artifact building

Use the base image defined by the Go workspace version. Set the
toolchain to the actual version. Therefore, in the Docker image, the
GOVERSION variable is set to our defined Go version (i.e., 1.25.0), and
we set the GOWORKSPACE (i.e., 1.25.1).

Signed-off-by: Ivan Valdes <ivan@vald.es>