Update prod build's baseURL to /unstable (#3035)
Update prod build's baseURL to /unstable (#3035)

Since we're deploying to https://spec.matrix.org/unstable now. This only updates the `baseURL` for our buildkite build. The default is still `/` when doing a local build.
matrix-org/matrix-doc,matrix-org/matrix-doc,matrix-org/matrix-doc,matrix-org/matrix-doc