Add nextclade3 via direct download

Nextclade v3 is not yet released in Bioconda. Add it as a direct
download so that users of Nextstrain's Conda runtime can transition to
using nextclade3.

src/build.sh

@@ -1,4 +1,10 @@
 #!/usr/bin/env bash
 set -euo pipefail -x
 mkdir -p "$PREFIX"/bin
+
+# envdir
 cp -v "$SRC_DIR"/envdir "$PREFIX"/bin/envdir
+
+# Nextclade v3
+curl -fsSL -o "$PREFIX"/bin/nextclade3 https://github.com/nextstrain/nextclade/releases/download/3.0.0-alpha.1/nextclade-$("$SRC_DIR"/target-triple)
+chmod a+rx "$PREFIX"/bin/nextclade3

src/target-triple

@@ -0,0 +1,23 @@
+#!/bin/bash
+#
+# Get the target triple from uname.
+#
+set -euo pipefail
+
+kernel="$(uname -s)"
+machine="$(uname -m)"
+
+case "$kernel/$machine" in
+    Linux/x86_64)
+        echo "x86_64-unknown-linux-gnu";;
+
+    Darwin/x86_64)
+        echo "x86_64-apple-darwin";;
+
+    Darwin/arm64)
+        echo "aarch64-apple-darwin";;
+
+    *)
+        echo "unsupported kernel/machine: $kernel/$machine" >&2
+        exit 1;;
+esac