Adding rust installation to the build script.

Normally this is done by the ./mach bootstrap command, but this is not ideal for the build script for a couple of reasons:

• Removing the --no-system-changes would also do this, but also install system packages which requires root privileges. We want to avoid needing a sudo command.
• By default, mach will install the latest version of rust, which may be too new by the time we get around to building. By installing ourselves, we can control the rust version which matches the firefox version (https://firefox-source-docs.mozilla.org/writing-rust-code/update-policy.html)

This patch maintains the rust version in the .PLAYWRIGHT_VERSION file and makes sure it gets installed.