ci(core): keep matrix jobs running after transient failures

Disable fail-fast for the Core build matrix so one flaky external
download does not cancel every other platform build. This preserves
successful matrix results and makes failed jobs independently
retryable.

.github/workflows/core.yml

@@ -58,7 +58,7 @@ jobs:
             easytier-web/frontend/dist/*
   build:
     strategy:
-      fail-fast: true
+      fail-fast: false
       matrix:
         include:
           - TARGET: x86_64-unknown-linux-musl