cd.yaml: do not use runner's os as cache key, since it seems buggy

.gitea/workflows/cd.yaml

@@ -12,7 +12,7 @@ jobs:
         with:
           path: |
             ~/.rustup/toolchains
-          key: ${{ runner.os }}-rust-toolchainv2
+          key: linux-rust-toolchainv2
       - name: Install minimal stable with clippy and rustfmt
         uses: actions-rs/toolchain@v1
         with:
@@ -39,7 +39,7 @@ jobs:
             ~/.cargo/registry/cache/
             ~/.cargo/git/db/
             target
-          key: ${{ runner.os }}-project-dependencies
+          key: linux-project-dependencies
       - run: cargo build --release --target x86_64-unknown-linux-gnu
       - name: Save cached cargo dependencies
         id: cache-project-dependencies-restore