add flag / env-var to select the probe by serial number

[japaric]

This is helpful when you want to cargo-run firmware on two or more different devices in parallel.

$ # terminal 1
$ cd path/to/firmware
$ PROBE_RUN_SN=abcdef012345 cargo run --bin radio-tx

$ # terminal 2
$ cd path/to/firmware
$ PROBE_RUN_SN=012345abcdef cargo run --bin radio-rx

In the above case the firmware can live in the same Cargo project.

When the firmware lives in different Cargo projects you can use the flag version.

$ # terminal 1
$ cd path/to/firmware-a
$ head .cargo/config
[target.thumbv7em-none-eabihf]
runner = "probe-run --serial-number abcdef012345 --chip nrf52"

$ cargo run --bin radio-tx

$ # terminal 2
$ cd path/to/firmware-b
$ head .cargo/config
[target.thumbv7em-none-eabihf]
runner = "probe-run --serial-number 012345abcdef --chip nrf52"

$ cargo run --bin radio-rx

[japaric]

also while implementing this, make probe-run fail if it finds 2 or more probes and it becomes ambiguous which one to use (no --serial-number flag was passed). (Right now probe-run picks the "first" probe).

[Lotterleben]

note from @jonas-schievink
Before attaching to a chip, the probe needs to be opened. probe-rs gives us a way to query all connected probes (Probe::list_all), and we currently pick the first one. This is what needs to be changed.