[devscout] bootstraps reproductible dev env

Our current build instructions request that the user install maven 3.6
but the build works just fine on the latest version.

On the other hand, they don't specify the version of the jdk to use to
 compile the project (jdk 21 is necessary).

With this, I propose to use the nix package manager (available on both
linux and max) to precisely pin versions of all of these tools for as
long as they are in the nix cache or their source is available.

In this file I didn't go as far as to pin to specific software version
except for the major jdk version. It is possible to do so if it becomes
necessary.