allow disabling RUSTFLAGS config

[JeanMertz]

The change in behaviour allows people to use this action, without overwriting their own rustflags configurations.

[jonasbb]

Hi, thanks for the PR. The rustflags handling of cargo is quite bad, so it makes sense to suppress setting RUSTFLAGS.

You deleted the previous logic of never overwriting RUSTFLAGS. Any particular reason for removing that check? Otherwise I think it makes sense to keep that. Either in general or maybe only as long as it is the default value. For example if you set RUSTFLAGS in your build matrix then those values should not simply be overriden.

[JeanMertz]

I don't mind adding it back, but the reason I removed it is two-fold:

1. Since there's now a rustflags option, that somewhat makes it obvious that if you keep it as the default, it'll set RUSTFLAGS.
2. But also, explaining what happens when became a bit more awkward if you keep both. As the default of the rustflags option is only applied if RUSTFLAGS isn't already set.

[jonasbb]

I think I rather keep the old behavior. Yes now there are two ways to set the RUSTFLAGS variable, but that seems ok.

But also, explaining what happens when became a bit more awkward if you keep both. As the default of the rustflags option is only applied if RUSTFLAGS isn't already set.

There are only three options. I would just describe it on a case by case basis.

1. RUSTFLAGS is already set. In that case, the variable is left untouched.
2. The variable is unset and rustflags input is empty. The variable remains unset.
3. Otherwise, set the environment variable RUSTFLAGS to the content of rustflags.

[jonasbb]

I took your PR and added back the behavior to not touch existing RUSTFLAGS variables. I wrote the changelog too.