breakpointHook: re-implement using nsenter

breakpointHook is amazing and has been a daily driver for me. Though it has been buggy several times in the past and recently it stopped working on my system, as the approach using cntr seems to be a bit sensitive to the environment and which shell is used etc.

After speaking with @mic92, it turned out there is an alternative approach using `nsenter`. After playing with that for a while, I think we should replace the current implementation of breakpointHook.

The new output when a build fails will look like this:
```
build failed in configurePhase with exit code 1
To attach, run the following command:
    sudo /nix/store/2kzihv08w9vsyi6zhs0pi2i9117v4q1v-attach/bin/attach 28715971284
```

Benefits of the new implementation over the old one:
- no need to install cntr tool on the local machine first, the copy pasted command will just work
- no need to execute `cntr exec ...` manually to enter the namespace
- interactive shell by default
- dropped into correct working directory by default
- $out available by default
- possibility to override the shell with zfs or fish, etc. by defining `debugShell` in the derivation to debug
- probably less prone to bugs as it only uses nsenter

cc @mic92

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