Switch to pinned conda Rust dependency

[charlesbluca]

For the sake of simplifying CI and the eventual work that may need to happen around the stdlib('c') migration, and especially considering it seems like an upcoming conda rust 1.78 might not play well with the current version of sysroot_linux-64 getting installed in CI environments, this PR attempts to pin us across CI where possible to one version of conda-forge's Rust - 1.77.