Deny warnings only in CI and xtask

[Dentosal]

Closes #1493

[xgreenx]

Using -D warnings for cargo make while not using the same RUSTFLAGS for regular build invalidates the building cache.

You can run ./ci_checks.sh twice a row, and you will see that it starts building from scratch.

[xgreenx]

I still think it is better to use !#[deny(warnings)] on the crate level because it doesn't cause the rebuilding of all dependencies, plus it can be easily disabled without rebuilding all dependencies. It works in all cases and covers only the crate that we are interested in.

We can introduce a new feature "ignore-warning" for each crate and use #![cfg_attr(not(feature = "ignore-warning"), deny(warnings))](but it will cause rebuilding of all dependencies if you want to ignore warnings, that also is bad)

[MitchTurner]

I agree that the warnings are annoying sometimes and I also don't like modifying the attributes during development--I'd end up forgetting to replace them.

I feel like overriding the lints via your cargo build command is a good solution. I haven't figured out how to get it to work yet though. This doesn't seem to do the trick:

RUSTFLAGS="-W warnings" cargo build

So I'm not sure if it's possible...

[xgreenx]

Closing for future investigation of how to make it work. More details in the issue #1493