Continue building after failing to install build-tool

Some packages refer to build-tools that are not cabal packages.
For example, "zip-archive" depends on "unzip", but this is talking about a binary, not a cabal package.
This is unfortunate, and perhaps the package shouldn't include this as it isn't a build tool, but we should probably provide some way to continue building regardless.

src/Mafia/Install.hs

@@ -379,12 +379,28 @@ installBuildTools :: CacheEnv -> Set BuildTool -> EitherT InstallError IO [Direc
 installBuildTools env tools = do
   pkgs <-
     for (Set.toList tools) $ \(BuildTool name constraints) ->
-      installPackage name constraints
+      tryInstall name constraints
 
   pure .
     fmap (</> "bin") .
     fmap (packageSandboxDir env) $
-    fmap pkgKey pkgs
+    fmap pkgKey $
+    catMaybes pkgs
+ where
+  -- Some packages refer to build-tools that are not cabal packages.
+  -- For example, "zip-archive" depends on "unzip", but this is talking about a binary, not a cabal package.
+  -- This is unfortunate, and perhaps the package shouldn't include this as it isn't a build tool, but we should probably provide some way to continue building regardless.
+  tryInstall name constraints = EitherT $ do
+    r <- runEitherT $ installPackage name constraints
+    case r of
+     Left e -> do
+      T.hPutStrLn stderr $
+        "Error while trying to install build tool '" <> unPackageName name <> "': "
+      T.hPutStrLn stderr $ renderInstallError e
+      T.hPutStrLn stderr "Trying to continue anyway, as some packages refer to non-existent build tools."
+      return $ Right Nothing
+     Right pkg -> do
+      return $ Right $ Just pkg
 
 -- | Detect the build tools required by a package.
 detectBuildTools :: Package -> EitherT InstallError IO (Set BuildTool)