fix: set h2 protocol identifier to comply with TLS-ALPN

[sgarcez]

In order to bump grpc-go (in this repo or in external clients) we need to specify h2 as the application protocol in buildkitd's TLS config.

The enforcement was introduced in grpc-go v1.67.0.

buildkit and buildx are currently on v1.66.2 so this patch is needed for the next update. I didn't update grpc-go in this patch, but can do if you prefer.

Testing with buildctl on v1.67.1

◊ cat go.mod | grep "google.golang.org/grpc\ "
        google.golang.org/grpc v1.67.1

Before this patch

◊ ./buildctl --addr tcp://localhost:1234 --tlsdir /tmp/certs/client/ build --local context=. --local dockerfile=. --frontend dockerfile.v0
[+] Building 0.0s (0/0)
error: listing workers for Build: failed to list workers: Unavailable: connection error: desc = "transport: authentication handshake failed: credentials: cannot check peer: missing selected ALPN property"

After patch

◊ ./buildctl --addr tcp://localhost:1234 --tlsdir /tmp/certs/client/ build --local context=. --local dockerfile=. --frontend dockerfile.v0
[+] Building 44.3s (71/71) FINISHED

Context

• https://github.com/grpc/grpc-go/releases/tag/v1.67.0
• TLS does not enforce ALPN protocol TLS does not enforce ALPN protocol  grpc/grpc-go#434
• credentials/tls: reject connections with ALPN disabled grpc/grpc-go#7184