ci: use remote ccache only, drop the duplicate local cache

Builds already point ccache at the shared Redis (CCACHE_REMOTE_STORAGE, set
per-runner by armbian/actions runner-clean). The local ccache directory is then
a second copy of the same objects, written to the disk that is the builders'
actual constraint: measured ~44 MB per 30 s per runner, against a device
showing 2.2 s average write latency and queue depth 178 under the nightly load.

CCACHE_REMOTE_ONLY is already supported by the ccache-remote extension and by
ccache 4.9.1 on the runners; it just was never passed. It has to go here rather
than in runner-clean's $GITHUB_ENV export, because docker.sh forwards a fixed
env allowlist into the build container and CCACHE_* is not on it -- which is
why CCACHE_REMOTE_STORAGE is passed as a compile.sh argument too.

Set inside the existing 'remote storage is configured' guard, so a build
without a remote keeps its local cache untouched.

Trade-off, stated because it is real: there is no local fallback. If Redis is
unreachable the cache is simply absent for that build and objects recompile.
CCACHE_REDIS_CONNECT_TIMEOUT is 500 ms, so that costs lookups, not a stall.

build-images-chunk.yml needs no change: it exports CCACHE_REMOTE_STORAGE to
$GITHUB_ENV but never passes ccache params to compile.sh, so image builds use
no ccache at all today. Kernel compiles happen in the artifacts path.

Signed-off-by: Igor Pecovnik <igor@armbian.com>

Author: igorpecovnik

.github/workflows/build-artifacts-chunk.yml

@@ -140,7 +140,12 @@ jobs:
           # If the runner exported a remote ccache storage, pass it on and enable the ccache-remote extension.
           declare -a CCACHE_PARAMS=()
           if [ -n "${CCACHE_REMOTE_STORAGE}" ]; then
-            CCACHE_PARAMS+=( "CCACHE_REMOTE_STORAGE=${CCACHE_REMOTE_STORAGE}" "ENABLE_EXTENSIONS=ccache-remote" )
+            # CCACHE_REMOTE_ONLY: with a shared Redis cache the local ccache dir is pure
+            # duplication -- measured ~44 MB per 30s per runner of writes to a disk that is
+            # the builders' bottleneck. Only set inside this guard, so a build without a
+            # remote keeps its local cache. Trade-off: no local fallback, so a Redis outage
+            # means cache misses (500 ms per lookup, CCACHE_REDIS_CONNECT_TIMEOUT), not a hang.
+            CCACHE_PARAMS+=( "CCACHE_REMOTE_STORAGE=${CCACHE_REMOTE_STORAGE}" "ENABLE_EXTENSIONS=ccache-remote" "CCACHE_REMOTE_ONLY=yes" )
           fi
           # Repeat on failure: retry the build up to BUILD_ATTEMPTS times to ride out transient
           # runner/network failures. Replaces the old external watchdog cron job.