Use nightly toolchain to build releases

Author: mmarx

.github/workflows/release.yml

@@ -25,3 +25,4 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }} # needs to be provided to change assets
           RUSTTARGET: ${{ matrix.target }}
           EXTRA_FILES: "README.md LICENSE-APACHE LICENSE-MIT"
+          TOOLCHAIN_VERSION: nightly