Disable nightly tests

[Urhengulas]

Since rustc 1.77.0-nightly the qemu-snapshot tests fail because the ip_in_core feature got stabilized.

error: the feature `ip_in_core` has been stable since 1.77.0-nightly and no longer requires an attribute to enable
  --> /home/runner/work/defmt/defmt/defmt/src/lib.rs:17:45
⏳ uninstalling targets
   |
17 | #![cfg_attr(feature = "ip_in_core", feature(ip_in_core))]
   |                                             ^^^^^^^^^^
   |
   = note: `-D stable-features` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(stable_features)]`

error: could not compile `defmt` (lib) due to 1 previous error

The proper solution is to remove the feature at our end (or make it a no-op for backcompat reasons). But at the moment the stable tests (rustc 1.75) require the feature attribute, but the nightly tests do not. Therefore we are disabling the nightly tests until rustc 1.77 is stable and are going to remove the feature attribute then and re-enable the nightly tests.