Set working RUSTFLAGS in .cargo/config.toml

[sveitser]

Closes #845

This PR:

Avoids developers running into compilation errors due to RUSTFLAGS.

The RUSTFLAGS in .cargo/config.toml will be used for all rustc invocations unless the RUSTFLAGS env var is set. In that case the env var will override the value in the config file.

Removes setting of --cfg hotshot_example (except for on the CI). This does lead to some warnings when compiling the tests with --release but requires less configuration. I think the main problem here is that we need to run tests with --release and we should instead fix that by for example enabling optimization of dependencies and leave the warning there until we fix it.

This PR does not:

Fix issues with compilation with tokio. Tracked in #1401

Key places to review:

It would be good if a vscode user could test if the changes to the settings are okay. cc @ImJeremyHe

[sveitser]

So somehow the static nix shell does not use the rustflags set in .cargo/config.toml :upside_down_face:

[sveitser]

This no longer needed, see #1856