CI workflow runs are ~10 mins

[joverlee521]

Noticed in testing nextstrain/conda-base#85 that the RSV pathogen-repo-ci job ran for ~10 mins unlike previous jobs that ran for ~3 mins. With the added time resolutions in #68, the CI now has to run 18 builds instead of 6 builds, so the math works out that the CI job runs ~3x longer.

We should update the CI job to only run a subset of builds or a single representative build for a more responsive CI job.
Some examples of other multi-build repos:

• Seasonal-flu runs a subset of builds (h3n2/ha/12y + h3n2/na/12y).
• avian-flu runs a single target build (h5n1/ha/all-time)