Update SLACK_CHANNELS envvar

With the latest update to the new Slack APIs for uploading files,
the `SLACK_CHANNELS` envvar is required to be a channel id instead
of a channel name.¹ Rather than hard-coding the channel id in the
workflows, I've made them GH Action environment variables that are
shared across the Nexstrain org so they can be shared with the
ncov repo.²

¹ <nextstrain/ingest#47>
² <https://github.com/organizations/nextstrain/settings/variables/actions>

.github/workflows/fetch-and-ingest-genbank-master.yml

@@ -95,7 +95,7 @@ jobs:
         CONFIG_OVERRIDES: ${{ needs.set_config_overrides.outputs.config_overrides }}
         GITHUB_RUN_ID: ${{ github.run_id }}
         NEXTSTRAIN_DOCKER_IMAGE: ${{ inputs.image || 'nextstrain/ncov-ingest' }}
-        SLACK_CHANNELS: ${{ inputs.trial_name && vars.TEST_SLACK_CHANNEL || 'ncov-genbank-updates' }}
+        SLACK_CHANNELS: ${{ inputs.trial_name && vars.TEST_SLACK_CHANNEL || vars.NCOV_GENBANK_SLACK_CHANNEL }}
       run: |
         nextstrain build \
           --aws-batch \

.github/workflows/fetch-and-ingest-gisaid-master.yml

@@ -94,7 +94,7 @@ jobs:
         CONFIG_OVERRIDES: ${{ needs.set_config_overrides.outputs.config_overrides }}
         GITHUB_RUN_ID: ${{ github.run_id }}
         NEXTSTRAIN_DOCKER_IMAGE: ${{ inputs.image || 'nextstrain/ncov-ingest' }}
-        SLACK_CHANNELS: ${{ inputs.trial_name && vars.TEST_SLACK_CHANNEL || 'ncov-gisaid-updates' }}
+        SLACK_CHANNELS: ${{ inputs.trial_name && vars.TEST_SLACK_CHANNEL || vars.NCOV_GISAID_SLACK_CHANNEL }}
       run: |
         nextstrain build \
           --aws-batch \