Do not error when adding an already installed channel. #604

Job	Run time
test-juliaup (x86_64-pc-windows-msvc)	9m 33s
test-juliaup (x86_64-pc-windows-gnu)	7m 52s
test-juliaup (x86_64-apple-darwin)	9m 54s
test-juliaup (x86_64-unknown-linux-gnu)	6m 13s
test-juliaup (x86_64-unknown-linux-musl)	6m 15s
test-juliaup (i686-pc-windows-msvc)	12m 17s
test-juliaup (i686-pc-windows-gnu)	13m 12s
check-juliaup (x86_64-pc-windows-msvc-windowsstore)	2m 50s
check-juliaup (x86_64-pc-windows-msvc-windowsappinstaller)	2m 27s
check-juliaup (x86_64-pc-windows-gnu-msi)	2m 49s
check-juliaup (x86_64-pc-windows-gnu-msi)	3m 47s
check-juliaup (x86_64-apple-darwin)	1m 40s
check-juliaup (x86_64-unknown-linux-gnu)	2m 59s
check-juliaup (x86_64-unknown-linux-musl)	2m 49s
check-juliaup (i686-pc-windows-msvc-windowsstore)	4m 32s
check-juliaup (i686-pc-windows-msvc-windowsappinstaller)	2m 44s
check-juliaup (i686-pc-windows-gnu-msi)	3m 27s
check-juliaup (i686-unknown-linux-gnu)	2m 17s
check-juliaup (i686-unknown-linux-musl)	2m 45s
check-juliaup (aarch64-unknown-linux-gnu)	2m 43s
check-juliaup (aarch64-unknown-linux-musl)	3m 34s
check-juliaup (aarch64-apple-darwin)	1m 40s
	1h 48m 19s

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