Empty Selector cache before running build pipeline

[mortenpi]

Small follow-up to #1440, to make sure that we would retain the feature where you can add selector stages between makedocs calls.

Just to make it concrete, with the current master the DummyBuildStage warning does not get printed if you run the following (but does work pre-#1440):

using Documenter: Documenter, makedocs, Selectors
makedocs(sitename = "foo")
abstract type DummyBuildStage <: Documenter.Builder.DocumentPipeline end
Selectors.order(::Type{DummyBuildStage}) = 2.5
Selectors.runner(::Type{DummyBuildStage}, args...) = @warn "DummyBuildStage"
makedocs(sitename = "foo")

@epatters I believe this should not affect the Catlab build times?

Close #1438

[fredrikekre]

I had written a comment suggesting this already in the original PR, but deleted it since it was easier to just press the button 😛

[mortenpi]

but deleted it since it was easier to just press the button

Somehow I immediately thought of this https://youtu.be/1OitCT1KT4k?t=158 (from 2:38) 😆

[epatters]

Thanks, can confirm that this does not affect the Catlab build time.