Stop rebuilding sdk-lints all the time (#2861)

I noticed `sdk-lints` was getting recompiled just about every time I ran
`git commit` as part of the pre-commit hooks. It looks like a compiler
flag was added to the `ExecRustBuildTool` task type that isn't actually
used by any tools. Removing it should eliminate compiler flag conflicts
between the gradle targets and pre-commit.

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buildSrc/src/main/kotlin/RustBuildTool.kt

@@ -28,7 +28,6 @@ private fun runCli(
  }
  }
  .copyTo(action)
- action.environment("RUSTFLAGS", "--cfg aws_sdk_unstable")
  action.execute()
  }
 }