ci: Drop nightly build/lint tests

There is not much point "preparing hassle-rs for the future" if the
lints are false positives and prevent us from submitting or having
actually good code.  Overnight two false positives got introduced:
https://github.com/Traverse-Research/hassle-rs/runs/5006913943?check_suite_focus=true
One needs the borrow to satisfy a peculiar `From`/`Into` implementation,
and the `&mut Vec<>` is pushed to which makes it impossible to turn this
into a mutable borrow of a slice.

.github/workflows/ci.yaml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest]
-        rust: [stable, nightly]
+        rust: [stable]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2