fix(solus): update Solus URLs

[silkeh]

Description

The latest Solus release has changed the naming of ISOs:

• Update all related functions to support the ISO name format used in the latest release.
• Use the directory listings in order to support more releases in the future.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] I have performed a self-review of my code
• [x] I have tested my code in common scenarios and confirmed there are no regressions
• [x] I have added comments to my code, particularly in hard-to-understand sections

[lj3954]

I think it may be a better idea to allow web_pipe to take in extra arguments for curl, rather than creating a separate function for a single header to be passed.

[philclifford]

@lj3954 I agree that a single more flexible function would be optimal. However this works and has no impact elsewhere so could be merged to fix the immediate issue of Solus breakage and a seperate issue created to refactor the two functions into one.