Tag each pathogen repo with the "nextstrain" and "pathogen" GitHub to…

…pics

A minor thing but the "pathogen" tag can help us easily list and search
within our pathogen repos on GitHub, e.g.

    https://github.com/search?q=org:nextstrain+topic:pathogen

We often use GitHub search to find code usages across repos or find
replicated issues/PRs, so having way to filter down to just pathogen
repos (vs. other software repos) is handy.

The "nextstrain" and "pathogen" topic tags are both in separate use more
widely¹, but not currently used together.²  There's a decent chance
Nextstrain community users will adopt the same tag pair by intentionally
following our example or by forking our repos, so we may become able to
use both tags to narrow down results when searching outside our org.
This would be neat for discovery.

An alternative I considered is using one topic, "nextstrain-pathogen",
but that seemed less desirable when considering that the existing
"nextstrain" and "pathogen" tags would also apply.

¹ <https://github.com/topics/nextstrain>
  <https://github.com/topics/pathogen>
² <https://github.com/search?q=topic:nextstrain+topic:pathogen>

env/production/github-repos.tf

@@ -0,0 +1,5 @@
+resource "github_repository_topics" "nextstrain" {
+  for_each = toset(flatten(values(local.pathogen_repos)))
+  repository = each.key
+  topics = ["nextstrain", "pathogen"]
+}