Verify Cabal functionality for various installs

This patch adds functionality to

* Test cabal build and cabal run in the CI workflow
* Test GHC 7.10.3 on Linux and ensures that it works
* On Linux, try hvr's PPA before ghcup

dist/index.js

@@ -8640,21 +8640,33 @@ async function installStack(version) {
 exports.installStack = installStack;
 async function installTool(tool, version) {
     core.startGroup(`Installing ${tool}`);
-    // Currently only linux comes pre-installed with some versions of GHC.
-    // They're intalled to /opt. Let's see if we can save ourselves a download
+    // Linux comes pre-installed with some versions of GHC and supports older
+    // versions through hvr's PPA.
     if (process.platform === 'linux') {
         // Cabal is installed to /opt/cabal/x.x but cabal's full version is X.X.Y.Z
         const v = tool === 'cabal' ? version.slice(0, 3) : version;
+        const p = path_1.join('/opt', tool, v, 'bin');
+        const installed = await fs_1.promises
+            .access(p)
+            .then(() => true)
+            .catch(() => false);
+        if (tool === 'ghc' && !installed) {
+            try {
+                // hvr's PPA has better support for GHC < 8.0
+                await exec_1.exec(`sudo -- sh -c "apt-get -y install ghc-${v}"`);
+            }
+            catch {
+                // oh well, we tried
+            }
+        }
         try {
-            const p = path_1.join('/opt', tool, v, 'bin');
             await fs_1.promises.access(p);
-            core.debug(`Using pre-installed ${tool} ${version}`);
             core.addPath(p);
             core.endGroup();
             return;
         }
         catch {
-            // oh well, we tried
+            // ok, let's try the generic install now
         }
     }
     if (process.platform === 'win32') {