Add option to update value of a config option

I think rbm could have a set-value command to update the value of a config option.

For example this command:

rbm/rbm set-value --target alpha --step build rust version 1.86.0

would try to update the first existing config option that it finds in projects/rust/config in the following order:

• steps/build/targets/alpha/version
• steps/build/version
• targets/alpha/version
• version

I think having this command in rbm would allow us to easily extend the list_toolchain_updates_checks scripts we have in tor-browser-build to also update the versions of components instead of just saying what needs to be updated.

/cc @pierov