Deny warnings in CI

[jessebraham]

Whew, this was a bit more challenging to figure out than expected 😅

Largely, the issue was that RUSTFLAGS are not cumulative, so it's not as simple as just specifying RUSTFLAGS=-Dwarnings, as this stomps out the configuration needed to actually build the HAL.

Thankfully I found a comment in a Cargo issue showing a workaround, which is to set the target-specific RUSTFLAGS, which are cumulative.

I was then able to use some bash-foo to transform the kebab-case target triple into SCREAMING_SNAKE_CASE, which is needed to construct the name of the required environment variable. We then set the environment variable for the target being built for to deny warnings.

Successful run:
https://github.com/jessebraham/esp-hal/actions/runs/10147479326

To verify it's working, I then introduced a warning, which caused CI to fail as expected:
https://github.com/jessebraham/esp-hal/actions/runs/10147731395/job/28058829410

Additionally, I noticed that at some point we (probably inadvertently) reverted to building with the nightly toolchain, so we're now building with stable again.

Closes #1797

[SergioGasquez]

LGTM! Thanks for sticking with this!

[JurajSadel]

LGTM, thanks for working on this!

[bjoernQ]

... the target-specific RUSTFLAGS, which are cumulative

Oh interesting - wasn't aware of that.

I wonder if we should/could just add the flag to .cargo/config.toml?

[jessebraham]

I wonder if we should/could just add the flag to .cargo/config.toml?

I think it would quickly become annoying, it's quite common to have a large number of warnings while working on new features and I don't think coercing those into hard errors gains us anything.