kernel-test.sh: stop defaulting to the other fork's build volume

BUILD_VOL defaulted to snaporca_buildcache — snaporca's volume — so running
this fork's kernel test wrote into the other fork's build cache. Defaults to
orcacad_kerneltest now.

docker-iter-build.sh had the identical defect and was fixed to
orcacad_buildcache; this script was missed at the time.

Observed rather than theorised: an orca_cad run under a different deps image
overwrote snaporca_buildcache's CMakeCache.txt, after which snaporca's own
kernel test failed to configure ("Cannot find NLopt library 'nlopt_cxx' in
.../lib/cmake/nlopt/lib") because it inherited the foreign cached paths. No
foreign object files were written — the run died at configure — but the cache
was poisoned, and the volume had to be wiped and rebuilt clean.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/kernel-test.sh

@@ -23,7 +23,10 @@ set -euo pipefail
 
 REPO="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
 IMAGE="${IMAGE:-snaporca-deps}"
-VOL="${BUILD_VOL:-snaporca_buildcache}"
+# Must NOT default to snaporca_buildcache: that is the other fork's volume, and pointing
+# this fork at it makes the two silently trade build artefacts. docker-iter-build.sh had the
+# identical defect and was fixed to orcacad_buildcache; this script was missed.
+VOL="${BUILD_VOL:-orcacad_kerneltest}"
 # Two pre-existing failures are excluded by default (see [known-broken] in
 # test_caddocument.cpp): one of them SIGABRTs inside the vendored solver and takes the
 # whole process down, so without this exclusion a green run is simply unreachable and the