fix(cache): support custom S3 CA certificates

Summary

• Add cache runtime support for S3_CA_CERT_PEM with a TUIST_S3_CA_CERT_PEM alias for parity with the server release.
• Route cache S3 Finch pool configuration through TuistCommon.FinchPools.s3_pool/1 so custom CA bundles are honored instead of always pinning CAStore.file_path().
• Add focused cache config and Finch pool tests covering the custom CA bundle path.
• Document the new variable in the cache self-hosting guide and wire it into the reference Docker Compose and deploy configs.

Testing

• mix test test/cache/config_test.exs test/cache/finch/pools_test.exs

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