fix: update explorer API endpoint for `Celo` / `CeloAlfajores` to use Etherscan #383

Job	Run time
clippy	18s
docs	17s
feature-checks	1m 4s
fmt	14s
miri --no-default-features	36s
wasm	22s
miri	38s
test stable --no-default-features	21s
miri --all-features	1m 0s
test stable	20s
test stable --all-features	29s
test beta --no-default-features	18s
test beta	19s
test beta --all-features	30s
test nightly --no-default-features	17s
test nightly	15s
test nightly --all-features	28s
test 1.79 --no-default-features	19s
test 1.79	20s
	8m 25s

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