solc is installed automatically by solcx. We should be able to remove the explicit solc install step from CI, but that causes the solium install to fail for unknown reasons. See #1452
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added 262 packages from 400 contributors in 8.544s
/home/runner/work/_temp/e28b13f5-0b56-49fb-b0b7-9c8c75c16ddd.sh: line 3: solium: command not found