DO-2027: Rewrite node-pr.yml to remove implicit skips

[tmthrgd-aligent]

This rewrites the node-pr.yml workflow primarily to remove the implicit
skips that can leave checks overlooked and not actually running. It also
simplifies several parts of the workflows and removes two additional
actions that were only used for this one workflow.

This may cause workflow breakage in projects. If failures are observed,
they were likely being masked by the now removed implicit skips. To fix
these failures, callers of the workflow should either add the necessary
explicit skips (i.e. add a 'skip-X: true' input) to the workflow or
correct the command being called by adding an 'X-command: command-name'
input to the workflow.

We now ignore two previously valid inputs, has-env-vars and fetch-depth.
Both inputs remain but are ignored. To remove them would require
updating all callers in lockstep.

has-env-vars will now be inferred from the presence of the ENV_VARS
secret and no longer needs to be set explicitly. This is consistent with
other inputs like pre-install-commands.

fetch-depth is removed as it never had an actual use-case and simply
duplicated the nx.json check the workflow already contained. It is used
by three callers, all of whom set it to 0 and all of which have an
nx.json file and will have a 'fetch-depth: 0' checkout anyway. We could
have instead kept the input and removed the implicit nx.json check, but
given it has been inconsistently used, this would also require a lot of
workflow updates. This can be re-added in the future if there was a
compelling need.

Ideally the two actions/setup-node steps would have been merged
together, but unfortunately actions/setup-node has no support for
running corepack enable which causes the cache setup step to error out.

Ideally we'd also merge the two actions/checkout steps into one, but
again, unfortunately we can't check for the presence of the nx.json file
until after we've performed an initial checkout. We could checkout just
the nx.json file and then perform the real checkout, but this would
likely take just as much work.

We are slightly more aggressive at performing a full checkout for Nx
projects than before. We now gate on simply the presence of an nx.json
file rather than checking whether a command we're going to run is
present in the nx.json file. This was the cause of frustration in
certain projects, is more straightforward and should not cause much
overhead. It should make our pipelines more robust for use in Nx-based
projects.

[tmthrgd-aligent]

I think this is ready for review now, unfortunately there are a few simplifications that couldn't be made for one reason or another (corepack and the double setup-node for example).

It's been tested against four repos: https://github.com/aligent/remote-cmd-bastion-hosting/pull/9, https://github.com/aligent/otr-int/pull/18, https://github.com/PolyaireDev/polyaire-frontend/pull/26 and https://github.com/aligent/aligent-applications-hosting/pull/14. All seem to be running the workflow correctly, but eyes across those would be good. If anyone has any other repos they'd like it tested against, let me know.

I don't think we'll have too many repos break from missing targets and the fix to them (adding the skip-X input is thankfully nice and simple).