feat: flag to select shell to use in adhoc scripts

Currently, spread assumes that the default shell is Bourne-like (perhaps specifically bash?) and runs adhoc scripts with whatever that default shell currently is. I recently changed my default shell from zsh to fish, and this broke a spread test for me.

Example:
```
multipass:
type: adhoc
allocate: |
# something that works in zsh, but not fish
COUNT="1"; echo $(($COUNT + 1))
# the fish equivalent for convenience
# set COUNT "1"; echo $(math $COUNT + 1)
```

Running a spread task with this config in zsh will work, but if your default shell is fish then you'll get fish errors:
```
++ COUNT="1"; echo $(($COUNT + 1))
fish: Unsupported use of '='. In fish, please use 'set COUNT "1"'.
COUNT="1"; echo $(($COUNT + 1))
^~~~~~~~^
```

I think it would be useful to have another parameter that could be set on these scripts:
```
multipass:
type: adhoc
shell: /usr/bin/bash
allocate: |
...
```

Start by tracing how spread parses task configuration and launches adhoc scripts. Check how the default shell is selected and where a per-task shell parameter could be applied. Done means an adhoc task can select its shell, while existing tasks retain current behavior and the shell-specific case is covered by tests.

Written by the indexing model from the issue text.