Switch MinGW building to CMake

Automatically grabs and compiles libpng, and avoids having hacky lines in our Makefile
(the compiler specification *should* be orthogonal to the build target!)

Also move the MinGW package install to the `install_deps.sh` script,
to move logic off of the YAML.

.github/workflows/coverage.yml

@@ -11,16 +11,13 @@ env:
 
 jobs:
   coverage:
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-latest
     steps:
       - name: Checkout repo
         uses: actions/checkout@v6
       - name: Install deps
         run: |
-          ./.github/scripts/install_deps.sh ubuntu
-      - name: Install LCOV
-        run: |
-          sudo apt-get install lcov
+          .github/scripts/install_deps.sh ubuntu-latest lcov
       - name: Install test dependency dependencies
         run: |
           test/fetch-test-deps.sh --get-deps ubuntu