bsd.sites.mk: use static CDN to download rust crates

static.crates.io is the recommended upstream download
path. This avoids "403 Forbidden" rate limiting results
when trying to download crates too fast. This also
seems to improve download speeds/latency.

As per https://crates.io/data-access#crate-content,

Crates can be downloaded directly from the crates.io CDN
[...]
No rate limits apply to static.crates.io at present.

And as per https://crates.io/data-access#api,

you are welcome to use the crates.io API provided you abide by the following limits:

• A maximum of 1 request per second, and
• A user-agent header that identifies your application.

Reviewed by: mikael
Differential Revision: https://reviews.freebsd.org/D59575