Example custom_metadata_url is flaky during CI testing

[lexnv]

 error: Failed to obtain metadata: Request error: Request timeout
 --> examples/examples/custom_metadata_url.rs:9:1
  |
9 | #[subxt::subxt(runtime_metadata_url = "wss://rpc.polkadot.io:443")]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this error originates in the attribute macro `subxt::subxt` (in Nightly builds, run with -Z macro-backtrace for more info)

error: could not compile `subxt-examples` due to previous error
warning: build failed, waiting for other jobs to finish...
Error: The process '/home/runner/.cargo/bin/cargo' failed with exit code 101

CI job: https://github.com/paritytech/subxt/actions/runs/3283295303/jobs/5407832664
PR: #694