runner.conda: Explicitly specify a nextstrain-base version when updating

This helps Micromamba (or really, libsolv) take the action we want,
upgrading to the latest version, as otherwise it sometimes chooses a
solution that results in no upgrade.¹

It's also good to be explicit about the version we expect, and this
behaviour is closer to what we do when updating other runtimes as well.
The update messaging is also improved in the no-op case by having the
desired version on hand.

¹ e.g. <#266 (comment)>

nextstrain/cli/runner/conda.py

@@ -52,7 +52,7 @@
 from ..errors import InternalError
 from ..paths import RUNTIMES
 from ..types import RunnerSetupStatus, RunnerTestResults, RunnerUpdateStatus
-from ..util import capture_output, exec_or_return, runner_tests_ok, warn
+from ..util import capture_output, colored, exec_or_return, runner_tests_ok, warn
 
 
 RUNTIME_ROOT = RUNTIMES / "conda/"
@@ -443,9 +443,44 @@ def update() -> RunnerUpdateStatus:
     """
     Update all installed packages with Micromamba.
     """
-    print("Updating Conda packages…")
+    current_version = (package_meta(NEXTSTRAIN_BASE) or {}).get("version")
+
+    # We accept a package match spec, which one to three space-separated parts.¹
+    # If we got a spec, then we need to handle updates a bit differently.
+    #
+    # ¹ <https://docs.conda.io/projects/conda/en/latest/user-guide/concepts/pkg-specs.html#package-match-specifications>
+    #
+    if " " in NEXTSTRAIN_BASE.strip():
+        pkg = PackageSpec.parse(NEXTSTRAIN_BASE)
+        print(colored("bold", f"Updating {pkg.name} from {current_version} to {pkg.version_spec}…"))
+        update_spec = NEXTSTRAIN_BASE
+
+    else:
+        latest_version = (package_distribution(NEXTSTRAIN_CHANNEL, NEXTSTRAIN_BASE) or {}).get("version")
+
+        if latest_version:
+            if latest_version == current_version:
+                print(f"Conda package {NEXTSTRAIN_BASE} {current_version} already at latest version")
+                print()
+                return True
+
+            print(colored("bold", f"Updating Conda package {NEXTSTRAIN_BASE} from {current_version} to {latest_version}…"))
+
+            update_spec = f"{NEXTSTRAIN_BASE} =={latest_version}"
+
+        else:
+            warn(f"Unable to find latest version of {NEXTSTRAIN_BASE} package; falling back to non-specific update")
+
+            print(colored("bold", f"Updating Conda package {NEXTSTRAIN_BASE} from {current_version}…"))
+
+            update_spec = NEXTSTRAIN_BASE
+
+    print()
+    print(f"Updating Conda packages in {PREFIX}…")
+    print(f"  - {update_spec}")
+
     try:
-        micromamba("update", NEXTSTRAIN_BASE)
+        micromamba("update", update_spec)
     except InternalError as err:
         warn(err)
         traceback.print_exc()