Launch secondary trial ncov builds using the up-and-coming new nextst…

…rain/base image

…instead of switching the primary ncov build to the new image, which
might be overly disruptive to the build shepherds.

The trial build will do all the things the primary build does, but
upload resulting files to a separate destination as not to interfere.
Build intermediates are uploaded under:

    s3://nextstrain-ncov-private/branch/new-build-base-image/…

While build outputs (JSONs) will be visible under:

    https://nextstrain.org/staging/trial/ncov/…

bin/launch-build

@@ -2,7 +2,6 @@
 set -euo pipefail
 
 nextstrain build --aws-batch --cpus 96 --memory 180GiB --detach \
-    --image nextstrain/base:branch-python-base \
     "$@" \
     --profile nextstrain_profiles/nextstrain \
     --set-threads tree=16

bin/rebuild-staging

@@ -66,6 +66,22 @@ main() {
 
     echo "Notifying Slack about rebuild."
     "$bin"/notify-slack "A new staging build was submitted. Follow along in your local ncov repo with: "'```'"nextstrain build --aws-batch --attach $aws_batch_job_id ."'```'
+
+
+    echo "Launching a trial build for nextstrain/base:branch-python-base"
+
+    # Must use a config file because Snakemake's --config options seemingly
+    # don't override the profile's config files as they should.  Not sure why.
+    tee trial.yaml <<<"
+---
+S3_BUCKET: nextstrain-ncov-private/branch/new-build-base-image
+s3_staging_url: s3://nextstrain-staging/trial_
+"
+
+    "$bin"/launch-build \
+        --image nextstrain/base:branch-python-base \
+        ncov deploy_to_staging upload \
+        --configfile trial.yaml
 }
 
 # Returns 1 if both files match (have identical hashes). Else returns 0.