Set MSRV in rust-toolchain.toml using the gradle.property (#3841)

The `rust-toolchain.toml` file for tests now uses the `rust.msrv` value
from the `gradle.properties` file.

This PR also fixes an issue where the `rust-toolchain.toml` file was not
created in the overridden test directory when `overrideTestDir` was set.
This caused the installed compiler version to be used, resulting in
errors with the latest compiler and preventing the use of
`overrideTestDir`.

Closes: #2048

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Co-authored-by: Fahad Zubair <fahadzub@amazon.com>