feat(ci): add Rust installation step for workflows

* Include Rust installation for Linux and macOS environments
* Ensures Rust is available for building dependencies

Author: saturneric

.github/workflows/testing-nightly.yml

@@ -143,8 +143,13 @@ jobs:
             mingw-w64-x86_64-texinfo
             mingw-w64-x86_64-libassuan
             mingw-w64-x86_64-ccache
+            mingw-w64-x86_64-rust
         if: runner.os == 'Windows'
 
+      - name: Install Rust
+        uses: dtolnay/rust-toolchain@stable
+        if: runner.os == 'Linux' || runner.os == 'macOS'
+
       - name: Build GpgME (macOS)
         run: |
           cd ${{github.workspace}}/third_party/gpgme