Build from source on >= v26.x rustc / cargo missing instructions & checks

[MikeMcC399]

The BUILDING.md document has incomplete instructions about temporal requirements.

For the v26.x and main branch, when building from source on Ubuntu:

WARNING: cargo not found! Support for Temporal will be disabled.
WARNING: rustc not found! Support for Temporal will be disabled.

and on Windows:

MSB8066: Custom build for 'Cargo.toml' exited with code 1

warnings and errors are emitted.

Suggestion

• Add any necessary new instructions and toolchain descriptions to BUILDING.md
• For Windows also add to prerequisite checks in vcbuild.bat

[MikeMcC399]

I'm closing this and I will resubmit separately for Linux and Windows