Respect shebang of task file

[jotaen]

Currently, the run.sh task file is interpreted by bash.

If run.sh is configured with #!/bin/zsh or something other, that setting is ignored, which might lead to unexpected behaviour.

The runner could support a number of different shells and automatically invoke them by parsing the shebang. There are a few things to keep in mind about this:

• The run.sh file format is not POSIX-compliant, due to the run:: prefix. /bin/sh or the like can therefore not be supported.
• It might be difficult to reliably determine the shell name from the shebang string:
  • The path is not canonical, so it can be /bin/bash, /usr/bin/env bash, /usr/local/bin/bash etc.
  • It might not be safe to rely on the name of the shell executable
• Shebangs are usually interpreted by the operating system, and so the behaviour is not standardised across OSses (see here for more reading). Not sure it’s a good idea or even possible to re-implement that properly.

So in the end, there might also be other approaches to solve this problem.