Modify some scripts to re-use them outside tor-browser-build

During the in-person meeting in Gothenburg, we said it might be useful to have some scripts that work also outside tor-browser-build (e.g., to inject tor and PTs in a browser directory).

So, we could extract part of our scripts and replace some templated parts with regular shell variables.

This would help as we could also lint scripts and look for major errors with automated tooling such as ShellCheck.

Another idea is that we could re-use some scripts as they are from Firefox's tree (e.g., the ones to build Clang or the wasi-sysroot), and reduce the maintenance burden on our side.

We could even have a scripts that takes an updated copy of those scripts from Mozilla as part of the toolchain update.