workflows/run-models: Unpin nextstrain-base image

[joverlee521]

Description of proposed changes

jax incompatibility issue in evofr¹ has been resolved in version 0.1.21 and the latest nextstrain-base image has been updated to include v0.1.21.²

¹ https://github.com/blab/evofr/issues/28 ² https://github.com/nextstrain/docker-base/actions/runs/6332834776

Related issue(s)

Resolves #57

Checklist

• [x] Checks pass
• [x] Test run

[joverlee521]

Test run completed successfully, outputs at s3://nextstrain-data/files/workflows/forecasts-ncov/trial/unpin-nextstrain-base/ look reasonable.