Split make linkcheck out into a distinct job [#106]

[genehack]

Description of proposed changes

Split make linkcheck into a distinct job, with continue-on-error: true, so that transient failures don't show as CI fails.

Related issue(s)

Closes #106

Checklist

• Checks pass

[victorlin]

This prevents linkcheck from running on conda-based build environments. It should be split into linkcheck-conda and linkcheck-pip.

Another approach would be what @tsibley proposed in #28 (review). This would allow for a single job to handle both conda and pip, but it's more complex:

My idea had been to make this workflow read the RTD config file and install what's configured there, which I still think would be nicer, but it's more complex.

[genehack]

This prevents linkcheck from running on conda-based build environments. It should be split into linkcheck-conda and linkcheck-pip.

I don't see the utility of running the exact same linkcheck process twice — that's just going to cause additional fails due to random network glitches.

In what scenario does a link resolve in the conda build and fail to resolve in the pip build, or vice versa?

[victorlin]

build-conda and build-pip are meant to be mutually exclusive, and the same would go for linkcheck-conda/linkcheck-pip. This is not checked in the workflow because it's reasonable to assume that a docs project will use only one. See GitHub query org:nextstrain nextstrain/.github/.github/workflows/docs-ci.yaml: some invocations specify environment-file while others specify pip-install-target.

[victorlin]

Closing this in favor of #110 as a solution to #106