Add some manual github actions to make building subsets easier

[NomDeTom]

Added 3 new workflows:

• Build Target List - makes a list of all of the targets within each arch
• Build One Arch - builds everything in a particular arch
• Build One Target - accepts a build target from the action above and builds it.

I've also added another conditional to one of the workflows to stop it running automatically on a fork.

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[DaneEvans]

Looks good, especially the changes to run only here.

I'm a bit concerned at the proliferation of additional pipelines, but manual only shouldn't pose an issue

[NomDeTom]

Looks good, especially the changes to run only here.

I'm a bit concerned at the proliferation of additional pipelines, but manual only shouldn't pose an issue

This is intended to only really be run on a forked repo, hence all the additional guards.

[fifieldt]

Thanks for this. The guards are good - having random actions run on forks is annoying :)

Q) These are nicely laid out. Is there a way they could be integrated into existing actions to avoid that little bit of duplication? (actually, is there any overlap?)

[NomDeTom]

@fifieldt I did consider rolling it all into the existing builder, but the actions to generate the matrix interferes with the generation of a single build target - the final build one target splits the action in two at this point. The build one arch and build target list are semi-superfluous, I suppose, but they're good demos of how their relevant actions work.