ci: pin CI jobs to the arm64 runners, not the shared ubuntu-latest alias

Same issue and fix as Guará: ci.yml's four jobs used
runs-on: ubuntu-latest, which now resolves only to hostinger-runner
after the Gitea runner relabeling, piling every CI push onto the same
single machine release.yml routes x86_64 builds to and overloading it.
Pins all four jobs to linux-arm64.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VAyr8d7HuAdUGB6aA5xmDD

.gitea/workflows/ci.yml

@@ -26,7 +26,7 @@ env:
 jobs:
   lint:
     name: Format and clippy
-    runs-on: ubuntu-latest
+    runs-on: linux-arm64
     env:
       CARGO_TARGET_DIR: /opt/hostedtoolcache/cargo-target/sabia-lint
     steps:
@@ -45,7 +45,7 @@ jobs:
   # Linux-only runner (ARM64).
   test:
     name: Tests (Linux)
-    runs-on: ubuntu-latest
+    runs-on: linux-arm64
     env:
       CARGO_TARGET_DIR: /opt/hostedtoolcache/cargo-target/sabia-test
     steps:
@@ -55,7 +55,7 @@ jobs:
 
   audit:
     name: Dependency audit
-    runs-on: ubuntu-latest
+    runs-on: linux-arm64
     steps:
       - uses: actions/checkout@v7
       - uses: dtolnay/rust-toolchain@stable
@@ -69,7 +69,7 @@ jobs:
 
   gui:
     name: Check desktop app (Linux)
-    runs-on: ubuntu-latest
+    runs-on: linux-arm64
     env:
       CARGO_TARGET_DIR: /opt/hostedtoolcache/cargo-target/sabia-gui
     steps: