.. `false`.

Currently, xplat-setup.yml passes on `shouldContinueOnError` to the
template with `true` if the parameter was `true`, else it sets the value
based on whether this is for runtime-staging, on a PR.

Which essentially means that runtime-staging defaults to
`shouldContinueOnError: true` for PRs, or we can set it explcitly to
`true` which would affect the rolling builds.

But this does not allow making a runtime-staging job *fail* when *tests
fail*. To support that this change adds a check for a special value
`forceFalse`, which causes the param to explicitly set to `false`. Thus
allowing `runtime-staging` builds to be marked as such.