Add cpu:2 tag to extractor tests (#11697)

This will limit the parallelism of extractor integration tests.

Each test runs now a bit faster although overall all tests seem
to run a bit slower. This seems a good compromise as it should
limit the contention over system resources between various
integration tests, each running a PostgreSQL instance and sharing
various system resources.\

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extractor/extractor.bzl

@@ -12,6 +12,7 @@ def extractor_test(name, srcs, flaky):
         name = name,
         flaky = flaky,
         size = "medium",
+        tags = ["cpu:2"],
         srcs = srcs,
         data = [
             "//daml-lf/encoder:testing-dar-latest",