lang/rust: Make OpenSSL use the system default truststore in Cargo
Cargo uses curl-rust and git2-rs (which uses curl-rest as well).
Unfortunately, git2-rs calls openssl_probe::init_ssl_cert_env_vars()
unconditionally which breaks the process environment by setting an invalid
value for SSL_CERT_DIR and then the system default truststore is circumvented,
resulting in certificate validation errors even if certctl(8) manages
everything nicely.
Upstream issues:
- https://github.com/alexcrichton/openssl-probe/issues/37
- https://github.com/rust-lang/git2-rs/pull/1130
Reviewed by: jrm (mentor), mikael (rust)
MFH: 2025Q1
Differential Revision: https://reviews.freebsd.org/D49120
(cherry picked from commit 078082660317490b1a2faf93ddce4b49c6b165dc)