Specify CI target in a Snakemake rule

Previously, updates to build names prompted tedious changes in
downstream CI usage, such as:

- nextstrain/augur@ef79e16
- nextstrain/docker-base@ebe2e55
- nextstrain/conda-base@2cda9b0

Now, such changes only require updates within this pathogen repo.

Co-authored-by: Jover Lee <joverlee521@gmail.com>

.github/workflows/ci.yaml

@@ -8,4 +8,4 @@ jobs:
   ci:
     uses: nextstrain/.github/.github/workflows/pathogen-repo-ci.yaml@master
     with:
-      build-args: auspice/avian-flu_h5n1_ha_all-time.json
+      build-args: test_target

Snakefile

@@ -15,6 +15,12 @@ rule all:
     input:
         auspice_json = all_targets()
 
+rule test_target:
+    """
+    For testing purposes such as CI workflows.
+    """
+    input: "auspice/avian-flu_h5n1_ha_all-time.json"
+
 rule files:
     params:
         dropped_strains = "config/dropped_strains_{subtype}.txt",