defer setup of WebChannels #880

Job	Run time
build	3m 1s
Julia 1.6 - ubuntu-latest - x64	3m 10s
Julia 1.6 - macOS-latest - x64	3m 31s
Julia 1.6 - windows-latest - x64	7m 23s
Julia 1 - ubuntu-latest - x64	5m 4s
Julia 1 - macOS-latest - x64	5m 9s
Julia 1 - windows-latest - x64	5m 44s
Julia pre - ubuntu-latest - x64	4m 51s
Julia pre - macOS-latest - x64	6m 30s
Julia pre - windows-latest - x64	5m 45s
Julia nightly - ubuntu-latest - x64	5m 10s
Julia nightly - macOS-latest - x64	6m 23s
Julia nightly - windows-latest - x64	5m 59s
	1h 7m 40s

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