Remove `editor` role from the test service account

Currently the service account used for our Kokoro builds has `editor` role. While this is convenient, strictly saying we should stop using the deprecated role and instead add specific minimum roles to the service account.

No file or test is named; start by locating the Kokoro build configuration and the service account's current IAM bindings. Determine the minimum roles required by the builds, remove the deprecated editor role, and verify that Kokoro builds still run with only those roles.

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