Make list_toolchain_updates provide a script to do version updates

After doing rbm#40091, I think it would be possible to update the list_toolchain_updates scripts to also generate a script to apply the toolchain updates that have been found.

The generated script would look something like that:

#!/bin/bash
rbm/rbm set-value --target alpha --step build rust version 1.92.0
rbm/rbm set-value --target alpha --step build nasm version 3.01
rbm/rbm set-value --target alpha --step build nasm var/nasm_sha256sum b7324cbe86e767b65f26f467ed8b12ad80e124e3ccb89076855c98e43a9eddd4

The steps to apply toolchain updates would be something like this:

• run make list_toolchain_updates
• review/edit the script out/firefox/apply-toolchain-updates-147.0a1.sh that has been generated
• run out/list_toolchain_updates/apply-updates-147.0a1.sh and get the changes applied to rbm config