ci: set FASTSENSE_SKIP_BUILD in release gate test runner

First install() compiles MEX. Second invocation skips rebuild
via env var, matching the main test workflow behavior.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: HanSur94

.github/workflows/release.yml

@@ -17,6 +17,8 @@ jobs:
       - uses: actions/checkout@v6
 
       - name: Build MEX and run tests
+        env:
+          FASTSENSE_SKIP_BUILD: "1"
         run: |
           octave --eval "install();"
           xvfb-run octave --eval "install(); cd('tests'); r = run_all_tests(); if r.failed > 0; exit(1); end"