test-lib: run ct_s2i_build_as_df code inside a subshell to avoid changing shell options
test-lib: run ct_s2i_build_as_df code inside a subshell to avoid changing shell options

The previous version with 'oldstate=$(set +o)' followed by 'eval "$oldstate"' seems
to have been setting "+e" to the caller of the function.

Required-by: postgresql-container#347

sclorg/container-common-scripts,sclorg/container-common-scripts