Convert the cache to golang

In the interest of performance, convert the fetcher and uploader to
golang and use the minio library, rather than repeated calls to the
server with the command line

nix/cache.nix

@@ -2,9 +2,10 @@
   writeShellApplication,
   active,
   distroFile,
+  buildGoModule,
+  lib,
 
-  curl,
-  minio-client,
+  go,
   packer,
 
   ...
@@ -12,36 +13,36 @@
 
 let
   inherit (builtins) concatStringsSep map;
+  bin = buildGoModule {
+    pname = "cache";
+    version = "0.0.0";
+    src = ./cache;
+    vendorHash = "sha256-Mtq6BIlBNxI28eoLPfayov5uUqC4gMK+4ib6VKjyAlU=";
+    #vendorHash = lib.fakeHash;
+    meta.mainProgram = "main";
+  };
 in
 writeShellApplication {
   name = "cache-isos";
   runtimeInputs = [
-    curl
-    minio-client
+    go
     packer
   ];
   text =
     ''
-      push="''${1:-yes}"
-      if [ "$push" == "yes" ]; then
-      	mc alias set storage "$STORAGE_URL" root "$MINIO_SECRET"
-      fi
-      mkdir -p cache
+      function list_isos {
     ''
     + (concatStringsSep "\n" (
       map (d: ''
-        url=$(echo var.iso.url | packer console -var-file="${distroFile d}" -config-type=hcl2 sources/)
-        sha=$(echo var.iso.checksum | packer console -var-file="${distroFile d}" -config-type=hcl2 sources/)
-        distro=$(echo var.distro | packer console -var-file="${distroFile d}" -config-type=hcl2 sources/)
-        # shellcheck disable=SC2143
-        if [ -z "$(mc ls "storage/isos/cache/$distro" | grep "$sha")" ]; then
-        	curl -L --retry 3 --retry-all-errors -o "cache/$sha.iso" "$url"
-        	if [ "$push" == "yes" ]; then
-        		mc put "cache/$sha.iso" "storage/isos/cache/$distro/$sha.iso"
-        	fi
-        else
-        	echo "Skipping ${d.distro} - cache/$sha.iso"
-        fi
-      '') active
-    ));
+        url=$(echo var.iso.url | packer console -var-file="${d}" -config-type=hcl2 sources/)
+        sha=$(echo var.iso.checksum | packer console -var-file="${d}" -config-type=hcl2 sources/)
+        distro=$(echo var.distro | packer console -var-file="${d}" -config-type=hcl2 sources/)
+        echo "$url" "$sha" "$distro"
+      '') (lib.unique (lib.map distroFile active))
+    ))
+    + ''
+      }
+
+      list_isos | ${lib.getExe bin}
+    '';
 }