Fix trial rebuilds

[joverlee521]

Description of proposed changes

Fix trial rebuilds for phylogenetic workflows by setting config overrides and sending trial build messages to the testing Slack channel (#scratch).

Related issue(s)

Follow up to

• https://github.com/nextstrain/mpox/pull/190
• https://github.com/nextstrain/mpox/pull/199

Checklist

• [x] Checks pass
• [x] Test mpxv trial bulid -> deployed to https://nextstrain.org/staging/trial/fix-trial-rebuilds/mpox/all-clades
• [x] Test hmpxv1 trial build -> deployed to https://nextstrain.org/staging/trial/fix-trial-rebuilds/mpox/clade-IIb
• [x] Test hmpxv1-big trial build -> deployed to https://nextstrain.org/staging/trial/fix-trial-rebuilds/mpox/lineage-B.1

[joverlee521]

Going to merge this pre-review since test trial builds all worked as expected. I will be using them for #238. Welcome to any post-merge review!