fix cmake in case of preinstalled grpc

[nfrmtk]

Relates: #266 #255
After fixing error in #266 with grpc -> gRPC I got new error about gRPC not being able to find Protobuf, fixed by adding line 49
And then generation started failing, so I've changed this and this

[sre-ci-robot]

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[sre-ci-robot]

Welcome @nfrmtk! It looks like this is your first PR to milvus-io/milvus-sdk-cpp 🎉

[nfrmtk]

/assign @yhmo

[nfrmtk]

/assign @matrixji

[matrixji]

/lgtm

[Mohamad-Hussein]

Thanks @nfrmtk , I've tried building with an existing gRPC installation before, and this solution worked for me!

[yhmo]

The code change has been merged to this pr: #273
I will close this pr, thank you for your contribution.