1.31.0 broke CircleCI build

[amaraliou]

Please see the FAQ in our main README.md, then answer the questions below before
submitting your issue.

What version of gRPC are you using?

1.31.0

What version of Go are you using (go version)?

1.10.3 (Version that comes with the machine executor in CircleCI, see here)

What operating system (Linux, Windows, …) and version?

Ubuntu 16.04 (again, comes with the machine executor in CircleCI)

What did you do?

Run this step as part of the CI workflow

- run:
    name: compile internal tools
    command: |
      export GOPATH=/home/circleci/go
      cd ~/go/src/github.com/dummy-profile/dummy-repo
      go get -v ./...
      go build ./tools/cmd/...; go install ./tools/cmd/...

What did you expect to see?

Things to build to completion

What did you see instead?

...
google.golang.org/grpc/serviceconfig
google.golang.org/grpc/internal
google.golang.org/grpc/internal/credentials
# google.golang.org/grpc/internal/credentials
../../../google.golang.org/grpc/internal/credentials/spiffe.go:34:70: state.PeerCertificates[0].URIs undefined (type *x509.Certificate has no field or method URIs)
../../../google.golang.org/grpc/internal/credentials/spiffe.go:38:47: state.PeerCertificates[0].URIs undefined (type *x509.Certificate has no field or method URIs)
../../../google.golang.org/grpc/internal/credentials/spiffe.go:56:35: state.PeerCertificates[0].URIs undefined (type *x509.Certificate has no field or method URIs)
google.golang.org/grpc/metadata
google.golang.org/grpc/internal/grpcrand
google.golang.org/grpc/codes
...

This issue is similar to #3775 with the difference that, instead of tls errors appearing, it's spiffe. I can already see that some stuff had been reverted for that, but spiffe seems to be still unchanged (see here)

[dfawley]

Go 1.10 is no longer supported by the latest releases of grpc-go. We only support the last three major releases of Go per our README. If you'd like to continue using an older version of Go, you will need to vendor grpc-go to an older version.