v8: update for changes to v8__String__WriteUtf8

https://github.com/lightpanda-io/zig-v8-fork/pull/211

.github/actions/install/action.yml

@@ -13,7 +13,7 @@ inputs:
   zig-v8:
     description: 'zig v8 version to install'
     required: false
-    default: 'v0.5.6'
+    default: 'v0.5.7'
   v8:
     description: 'v8 version to install'
     required: false

.github/actions/v8-snapshot/action.yml

@@ -13,7 +13,7 @@ inputs:
   zig-v8:
     description: 'zig-v8 release tag the prebuilt lib came from'
     required: false
-    default: 'v0.5.6'
+    default: 'v0.5.7'
 
 runs:
   using: "composite"