runner.conda: Allow downgrades during update of nextstrain-base

This permits Micromamba to downgrade packages that are already installed
in order to upgrade nextstrain-base.  While nextstrain-base's
fully-locked dependencies from version to version will typically only
cause upgrades, there are times when downgrades may be required.¹  When
that's the case, Micromamba simply won't consider that nextstrain-base
version as available without --allow-downgrade.

Resolves <#264>.

¹ For example, the deps of Nextstrain CLI's package in Bioconda changed
  to better match its deps in PyPI², which meant versions of botocore,
  boto3, and some related packages had to be downgraded.

² <bioconda/bioconda-recipes#39711>

nextstrain/cli/runner/conda.py

@@ -305,6 +305,13 @@ def micromamba(*args, add_prefix: bool = True) -> None:
             # Don't automatically pin Python so nextstrain-base deps can change
             # it on upgrade.
             "--no-py-pin",
+
+            # Allow uninstalls and downgrades of existing installed packages so
+            # nextstrain-base deps can change on upgrade.  Uninstalls are
+            # currently allowed by default (unlike downgrades), but make it
+            # explicit here.
+            "--allow-uninstall",
+            "--allow-downgrade",
         )
 
     env = {