ci: remove mstksg/get-package

This action has not been updated since 2019, and is currently causing
some deprecation warnings to show up. This replaces it with a few run
steps for each OS.

.github/workflows/ci.yml

@@ -17,18 +17,17 @@ jobs:
         os:
           - ubuntu-20.04
           - macos-latest
-        include:
-          - os: ubuntu-20.04
-            apt-get: autoconf automake libtool
-          - os: macos-latest
-            brew: automake
 
     steps:
-      - name: Get Packages
-        uses: mstksg/get-package@v1
-        with:
-          brew: ${{ matrix.brew }}
-          apt-get: ${{ matrix.apt-get }}
+      - name: Get Packages (macOS)
+        if: runner.os == 'macOS'
+        run: brew install automake
+
+      - name: Get Packages (Linux)
+        if: runner.os == 'Linux'
+        run: |
+          apt-get -q update
+          apt-get -q install autoconf automake libtool
 
       - name: Checkout
         uses: actions/checkout@v3