Pin some CI jobs to specific rust versions

Unfortunately, some new versions may change behavior, e.g. clippy may
add lints, which break the build. Therefore it makes sense, to pin
certain error prone CI-jobs to some specific versions. They should be
bumped regularly, though.

Currently, the following jobs are pinned:
1. the `ci/test`-job: as shown in #105, Rust 1.83 has introduced a valid
   "leak", which fails our tests for now. Therefore this job is pinned
   to 1.82, where this issue doesn't occur. A proper fix is necessary in
   the future (#107).
2. `clippy`: the latest [PR broke the clippy job][clippy-job] due to new
   lints being fired. Therefore the CI is pinned to 1.83 with the issues
   fixed.

[clippy-job]: https://github.com/jfrimmel/cargo-valgrind/actions/runs/12095283463/job/33728289241

.github/workflows/ci.yaml

@@ -26,5 +26,6 @@ jobs:
       - uses: actions/checkout@v4
       - name: Install valgrind
         run: sudo apt install valgrind
+      - run: rustup update --no-self-update 1.82 && rustup default 1.82
       - name: Run tests
         run: cargo test

.github/workflows/style.yaml

@@ -20,7 +20,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - run: rustup update stable && rustup default stable
+      - run: rustup update --no-self-update 1.83 && rustup default 1.83
       - run: rustup component add clippy
       - name: Run linter
         run: cargo clippy --color=always -- -D warnings