Fix removing gRPC in Dockerfile (#172)

We are installing using `pip install --target`. To fix this we must set
PYTHONPATH so pip can find and uninstall these packages.

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Dockerfile

@@ -16,6 +16,7 @@ RUN mkdir workspace && pip install --target workspace ./aws-opentelemetry-distro
 # will complain that grpc is not installed, which is more understandable. References:
 # * https://github.com/open-telemetry/opentelemetry-operator/blob/b5bb0ae34720d4be2d229dafecb87b61b37699b0/autoinstrumentation/python/requirements.txt#L2
 # * https://github.com/MicrosoftDocs/azure-docs/blob/main/articles/azure-functions/recover-python-functions.md#troubleshoot-cannot-import-cygrpc
+ENV PYTHONPATH=/operator-build/workspace
 RUN pip uninstall opentelemetry-exporter-otlp-proto-grpc -y
 RUN pip uninstall grpcio -y