Add a missing apt-get update before install (#48)

I always forget this and it always bites us within a few months. Alas.

Author: alexcrichton

.github/workflows/main.yml

@@ -588,7 +588,7 @@ jobs:
       if: ${{ matrix.cross }}
     - name: Install apt packages
       if: ${{ matrix.apt_packages }}
-      run: sudo apt-get install -y ${{ matrix.apt_packages }}
+      run: sudo apt-get update && sudo apt-get install -y ${{ matrix.apt_packages }}
     - run: ${{ matrix.test }}
       env:
         CARGO_BUILD_TARGET: ${{ matrix.target }}