ci: turn on tcp feature in examples

[lwshang]

When I was testing the x86_64 example with tcp feature, I got a compilation error at the line of dev.transmit(tx_buf).unwrap();.

The error was introduced in a recent PR: #111.
It was overlooked by CI because the default value tcp=off was in effect.

I found the original rationale for disabling tcp in CI from this commit message: 5b0786b

Disable tcp feature in x86_64 example by default.
It waits forever, which is no good in CI.

So I modify the test_echo_server function to break out the dead loop.

I also applied the same fix to the riscv example.

[lwshang]

GitHub CI has rustup by default, it can pickup the correct toolchain from rust-toolchain.toml files automatically.

So there is no need to setup toolchain in main.yml anymore. The toml files will be the only source of truth.

[qwandor]

Thanks!