Make sure examples can actually build with defmt enabled, build some examples in CI

[bugadani]

This PR:

• removes duplicate example checks
• adds stuff to make sure the defmt feature not only passes cargo check but actually builds
• adds +nightly toolchain spec for consistency
• don't enable esp-println/log by default
• fixes examples to work with log disabled

[bugadani]

Turns out, many of the current examples (that are checked by CI) don't actually link. Part of the problem is defmt, which is fixed by this PR (though I'm still very much confused why e-h-common can build at all, even though Interrupts doesn't implement Format yet...?), but other than this, the timer and sw interrupt examples aren't linking if embassy support is enabled, due to multiple definitions of certain interrupt handlers. How should we handle these examples?

[bjoernQ]

looks good to me but I'd like to leave the final word on this to @jessebraham since he did most of the work on CI

[jessebraham]

Sorry for the delay in getting to this, thanks for making that change. LGTM!

[bugadani]

Hold on please, things don't build locally.

[jessebraham]

At this point I'm getting pretty concerned that we've added this defmt feature and nobody can seem to get CI to verify it.

[bugadani]

This PR wasn't trying to get CI to verify the feature, but I can add a cargo build for each build target if you want me to.

[jessebraham]

The title of the PR is Make sure examples can actually build with defmt enabled, CI is green, and you're saying it's not building. Maybe I'm misunderstanding but this doesn't seem right.

[bugadani]

Yeah, this is a fix to build the examples locally. I didn't want to increase CI times by turning some cargo checks into cargo builds.

[bugadani]

@jessebraham if you're fine with cargo build this is fine now. Looks like the linker script needs to be added to each package. I'm suspecting something's up with conditional compilation and build scripts.