Teach classic snaps to select the right interpreter

[Evan]

The naive way of writing a snapcraft.yaml for a classic nodejs snap:

```
name: hiper
base: core18
version: git
summary: A statistical analysis tool for performance testing
description: |
  A statistical analysis tool for performance testing.

grade: stable
confinement: classic

parts:
  hiper:
    plugin: nodejs
    source: .

apps:
  hiper:
    command: hiper

```

This will spin forever as the shebang in bin/hiper (#!/usr/bin/env node) will point at the outer nodejs. Instead, the apps section should be written as:

```
apps:
  hiper:
    command: bin/node $SNAP/bin/hiper

```

This is not obvious. I think wanting to use the interpreter from inside the snap is more common than wanting to use the one from outside. Can we rewrite the shebangs accordingly?

[Sergio Schvezov (sergiusens)]

https://github.com/snapcore/snapcraft/pull/2560