CI: don't test oldtime feature

Author: pitdicker

.github/workflows/test.yml

@@ -76,7 +76,7 @@ jobs:
       - uses: Swatinem/rust-cache@v2
       - run: |
           cargo hack check --feature-powerset --optional-deps serde,rkyv \
-            --skip __internal_bench,iana-time-zone,pure-rust-locales,libc,winapi \
+            --skip __internal_bench,iana-time-zone,pure-rust-locales,libc,oldtime,winapi \
             --all-targets
         # run using `bash` on all platforms for consistent
         # line-continuation marks