Switch to Cargo instruction compatible with older versions of Cargo

[arctic-alpaca]

Fixes #837

[Urhengulas]

@arctic-alpaca Will this work forever or will this syntax be deprecated in favor of the new one at some point? If it is gonna be deprecated we should add a not or a tracking issue.

[arctic-alpaca]

This is indeed deprecated, but required for older versions of cargo to function:
Note: The old invocation prefix cargo: (one colon only) is deprecated and won’t get any new features. To migrate, use two-colons prefix cargo::, which was added in Rust 1.77. If you were using cargo:KEY=VALUE for arbitrary links manifest key-value pairs, it is encouraged to switch to cargo::metadata=KEY=VALUE. Stick to cargo: only if the support of Rust version older than 1.77 is required.
(source)

This PR brings the newly added cargo:rustc-check-cfg instruction in line with other instructions already present, that all use the deprecated invocation prefix.

For backwards compatibility while migrating to the new prefix, a feature could be added to switch to the old prefix for those stuck on older toolchain versions.

[Urhengulas]

I am fine with using the old syntax for now, also because it is consistent with the other invocations as you pointed out. We need to re-evaluate as soon as we need a new feature which only supports ::. Until then I think we are fine like this.