fetch-configlet: support fetching arm64 assets (#805)

The most recent configlet release [1] includes pre-built binaries for:

- arm64 Linux [2]
- arm64 macOS [3]
- arm64 Windows [4]
- riscv64 Linux [5]

Support arm64 in the bash fetch-configlet script.

Refs: #122

[1] https://github.com/exercism/configlet/releases/tag/4.0.0-beta.14
[2] 0e8d665, 2023-08-16, ".github, config: use Zig to cross-compile arm64 Linux asset"
[3] f280445, 2023-08-17, ".github: cross-compile arm64 macOS asset"
[4] 3824299, 2023-08-18, ".github, cli, completion: cross-compile arm64 Windows asset"
[5] a962b18, 2023-08-17, ".github: cross-compile riscv64 Linux asset"

scripts/fetch-configlet

@@ -24,10 +24,11 @@ get_download_url() {
   local latest='https://api.github.com/repos/exercism/configlet/releases/latest'
   local arch
   case "$(uname -m)" in
-    x86_64) arch='x86-64' ;;
-    *686*)  arch='i386'   ;;
-    *386*)  arch='i386'   ;;
-    *)      arch='x86-64' ;;
+    aarch64|arm64) arch='arm64'  ;;
+    x86_64)        arch='x86-64' ;;
+    *686*)         arch='i386'   ;;
+    *386*)         arch='i386'   ;;
+    *)             arch='x86-64' ;;
   esac
   local suffix="${os}_${arch}.${ext}"
   curl "${curlopts[@]}" --header 'Accept: application/vnd.github.v3+json' "${latest}" |