ci: add tsan spack environment with instrumented libzmq

- mirror spack-latest.yaml, with -fsanitize=thread on the libzmq and
  libsodium nodes so tsan can observe the happens-before edges established
  inside libzmq's lock-free queues, plus the libstdcxx-tsan root spec
- flags are applied per node instead of via the propagating '==' operator,
  which could reach the gcc node and trigger a compiler rebuild
- unchanged roots (fairlogger, boost, ninja, cmake) keep their spec hashes,
  so they are shared with the regular buildcache entries; the instrumented
  nodes hash differently and coexist in the content-addressed cache
- exclude libstdcxx-tsan from concretizer reuse so recipe changes always
  take effect; unchanged recipes still hit the buildcache because the spec
  hash is identical
- add the tsan env to the buildcache matrix (rebuilding also on spack_repo
  changes) so the instrumented binaries are cached instead of rebuilt on
  every CI run

.github/workflows/buildcache.yml

@@ -8,6 +8,7 @@ on:
     branches: [dev, master]
     paths:
       - 'test/ci/spack-*.yaml'
+      - 'test/ci/spack_repo/**'
       - '.github/workflows/buildcache.yml'
       - '.github/actions/setup-deps/**'
 
@@ -30,6 +31,8 @@ jobs:
         include:
           - gcc: '15'
             env: 'boost187'
+          - gcc: '15'
+            env: 'tsan'
 
     steps:
       - uses: actions/checkout@v6