main doesn't build with bridges but no PTs

cargo check -p arti-client --features=bridge-client

    Checking tor-guardmgr v0.7.0 (/home/ian/Rustup/Arti/arti/crates/tor-guardmgr)
error[E0004]: non-exhaustive patterns: `&ChannelMethod::Pluggable(_)` not covered
   --> crates/tor-guardmgr/src/bridge/config.rs:341:30
    |
341 |         let settings = match addrs {
    |                              ^^^^^ pattern `&ChannelMethod::Pluggable(_)` not covered
    |
note: `ChannelMethod` defined here
   --> /home/ian/Rustup/Arti/arti/crates/tor-linkspec/src/transport.rs:407:5

Problems:

1. this is due to arti-client unconditionally depending on tor-ptmgr which unconditionally enables the pt-client feature of tor-linkspec. Then we have tor-linkspec/pt-client but tor-guardmgr/!pt-client which is fail.

2. I think adding variants to an exhaustive enum is not cargo-feature-additive. So I think we need to make ChannelMethod non-exhaustive which will be annoying.

3. we apparently never try to build with bridges but not PTs in CI.