chore(ci): run tarpaulin in release mode

This is a small experiment: it turns out compiling is relatively quick,
on a PR I just did (3 minutes), but running the tarpaulin build took
around 8 minutes. I'm wondering if increasing the compile times and
lowering the run time is worth it, hence this experiment.

.github/workflows/coverage.yml

@@ -93,7 +93,7 @@ jobs:
       - name: Run tarpaulin
         run: |
           rustup run stable cargo tarpaulin \
-            --skip-clean --profile cov --out xml \
+            --release --skip-clean --profile cov --out xml \
             --features experimental-widgets,testing
         env:
           CARGO_PROFILE_COV_INHERITS: 'dev'