CI: add retries and package caching across all platforms

- Add retry loops to all package manager commands (apt, apk, pacman,
pkg).
- Implement daily package caching to minimize external server hits.
- Enable Docker layer caching in CI using buildx and GHA backend.
  - Sync CI Docker build steps with the build-with-docker.sh logic.
  - We don't, or more accurately, can't actually use
`build-with-docker.sh` anymore in CI. Maybe we should move it to
scripts/tools/ to keep the root directory clean.
- add ramdisk for BSD, shave 30/60 seconds.
- change BDSs to cross-platform-actions/action for a cleaner ci and skip
SLOW_TESTS for BSDs: this shaves a few seconds
- set global CACHE_INVALIDATION_SEED

scripts/tools/ci_tests.sh

@@ -207,7 +207,12 @@ run_wasm_compile() {
 run_unit_tests() {
     echo "--- Running Unit Tests ---"
     cd "$ROOT_DIR/test"
-    "$C3C_BIN" compile-test unit -O1 -D SLOW_TESTS
+
+    UNIT_TEST_ARGS="-O1"
+    if [[ "$OS_MODE" != "bsd" ]]; then
+        UNIT_TEST_ARGS="$UNIT_TEST_ARGS -D SLOW_TESTS"
+    fi
+    "$C3C_BIN" compile-test unit $UNIT_TEST_ARGS
 
     echo "--- Running Test Suite Runner ---"
     "$C3C_BIN" compile-run -O1 src/test_suite_runner.c3 -- "$C3C_BIN" test_suite/ --no-terminal