Automatically upload builds from PRs to a temporary label

As discussed with @jakirkham in https://github.com/conda-forge/hdf5-feedstock/pull/28#issuecomment-238109575, it would be nice if the builds produced by the CI as part of the testing of PRs could get uploaded to the conda-forge channel under a temporary label. This would allow people to test out a build locally or even as part of other builds, or do other kinds of more extensive testing, before deciding to go ahead and merge the PRs into master.

The label names would have to be unique, so one possibility would be to adopt the convention `pr--`, for instance `pr-hdf5-28`. The label would then always contain the latest build from a PR (so we'd need to upload and overwrite previous builds), and when a PR is merged, we could make it so that it deletes the label.

I think it would be better to have unique labels such as this, because otherwise if we had a single `test` or `staging` label, it would be hard to isolate which package local issues were due to.