CMake+Rust: Fix offline builds w. vendored git repos

Some builds using the tarball with the vendored Rust dependencies are
failing.

The onenote-rs dependency is presently tied to a git branch from github
rather than using a release from crates.io. This is the differing factor
though I'm unsure why it is causing the build to try to update the repo
rather than just building the vendored source.

This commit adds a `--offline` parameter to the build options if the
vendored source is detected, in an attempt to force Cargo to use what it
has and stay offline.

cmake/FindRust.cmake

@@ -460,6 +460,10 @@ endif()
 
 set(CARGO_ARGS "build")
 
+if(EXISTS "${ARGS_SOURCE_DIRECTORY}/.cargo/vendor")
+    list(APPEND CARGO_ARGS "--offline")
+endif()
+
 if(NOT "${RUST_COMPILER_TARGET}" MATCHES "^universal-apple-darwin$")
     # Don't specify the target for macOS universal builds, we'll do that manually for each build.
     list(APPEND CARGO_ARGS "--target" ${RUST_COMPILER_TARGET})