Sync all: use GitHub Actions matrix

[ewels]

I've never really liked the --all flag for the nf-core sync command. It would be much nicer if we could use GitHub Actions to parallelise the synchronisation of all pipelines and keep the nf-core/tools command for just one.

I think that this should be possible, by using some of the JSON parsing functionality from GitHub actions in a step, setting things as environment variables and saving these for a subsequent job to use. That will need to depend on the first job so that it waits, and can then use the environment variable list as a matrix argument.

[ewels]

Finally got a proof of concept working at https://github.com/ewels/nf-core-tools/runs/929776157

[ewels]

Done in #716