There is known issue where PowerShell is unreasonably slow downloading
files due to an issue with rendering the progress bar, see:
https://github.com/PowerShell/PowerShell/issues/2138
That issue is fixed in PowerShell Core (available in Azure Pipelines as
pwsh.exe) but it can also be worked around by setting:
$ProgressPreference = 'SilentlyContinue'
This commit is intended to go through and review/refactor the azure
pipelines configuration we have. The major changes are:
* The separate `{windows,macos,linux}.yml` files are now all merged into
one `run.yml`. This allows a shared "master flow" for all platforms
with divergence only where necessary.
* Some install steps have been separated as `install-*.yml` scripts,
where each script internally matches on the appropriate OS and then
delegates accordingly.
* Some various bits and pieces of cruft have been removed which were
artifacts of Travis's setup or similar.
