azure: `sccache` not found

The Azure pipeline is currently failing on my PR because it is [unable to find `sccache`](https://dev.azure.com/rust-lang/rust/_build/results?buildId=606&view=logs&jobId=d5810e3a-0d17-5d9d-3d2e-f99007e94ae9&taskId=e50ec509-f79e-5c61-3d86-4e0717295716&lineStart=136&lineEnd=136&colStart=1&colEnd=42).