ci: gate validate jobs on github.server_url instead of runner.arch

The runner.arch context isn't populated for job-level if conditions
on Gitea Actions, so the previous guard didn't skip on ARM64 Gitea
runners. Switch to checking github.server_url, which is reliably
'https://github.com' on GitHub-hosted runs and the Gitea host
everywhere else.

.github/workflows/validate.yml

@@ -16,7 +16,7 @@ jobs:
   hassfest:
     name: Hassfest validation
     runs-on: ubuntu-latest
-    if: runner.arch != 'ARM64'
+    if: github.server_url == 'https://github.com'
     steps:
       - uses: actions/checkout@v4
       - uses: home-assistant/actions/hassfest@master
@@ -24,7 +24,7 @@ jobs:
   hacs:
     name: HACS validation
     runs-on: ubuntu-latest
-    if: runner.arch != 'ARM64'
+    if: github.server_url == 'https://github.com'
     steps:
       - uses: actions/checkout@v4
       - uses: hacs/action@main