upload-to-s3: Use GNU coreutils sha256sum

The GNU coreutils `sha256sum` is much faster than the custom Python
implementation in this repo. We no longer have to worry about
portability since GN coreutils has been added to our conda runtime.¹

¹ <nextstrain/conda-base#70>

upload-to-s3

@@ -25,7 +25,7 @@ main() {
     local key="${s3path#*/}"
 
     local src_hash dst_hash no_hash=0000000000000000000000000000000000000000000000000000000000000000
-    src_hash="$("$bin/sha256sum" < "$src")"
+    src_hash="$(shasum "$src")"
     dst_hash="$(aws s3api head-object --bucket "$bucket" --key "$key" --query Metadata.sha256sum --output text 2>/dev/null || echo "$no_hash")"
 
     if [[ $src_hash != "$dst_hash" ]]; then
@@ -75,4 +75,9 @@ content-type() {
     esac
 }
 
+shasum() {
+    # Grab just the hex digest, ignoring the echoed input filename
+    sha256sum "$@" | cut -d ' ' -f 1
+}
+
 main "$@"