ci(test): set DYLD_SHARED_REGION=avoid on macOS to dodge dyld shared-cache flake

GitHub macos-15-arm64 runners intermittently kill pre_process at launch
with 'dyld cache (null) not loaded: syscall to map cache into shared region
failed' (signal 9 / exit 137), failing ~13% of launches on affected VMs.
DYLD_SHARED_REGION=avoid makes dyld map dylibs privately instead of via the
shared region, bypassing the failing syscall. No-op on Linux.

Author: sbryngelson

.github/workflows/test.yml

@@ -275,6 +275,12 @@ jobs:
           TEST_PCT: ${{ matrix.debug == 'reldebug' && '-% 20' || '' }}
           PRECISION: ${{ matrix.precision != '' && format('--{0}', matrix.precision) || '' }}
           CHANGED_FILES: ${{ needs.file-changes.outputs.changed_files }}
+          # Workaround for an intermittent GitHub macos-15-arm64 runner-image flake
+          # where pre_process dies at launch with "dyld cache '(null)' not loaded:
+          # syscall to map cache into shared region failed" (signal 9). Telling dyld
+          # to avoid the shared region makes it map dylibs privately, sidestepping the
+          # failing syscall. No-op on Linux (empty value, no dyld).
+          DYLD_SHARED_REGION: ${{ matrix.os == 'macos' && 'avoid' || '' }}
 
       # ── NVHPC build + test (via docker exec into long-lived container) ──
       - name: Build (NVHPC)