engine: fix provider config precedence

Regression from coreos#958. We switched the list of providers from an array to
a map. But iteration order through a map is undefined, so we lost the
precedence of providers.

I think this is the cause behind a lot of the FCOS installer test
timeouts, such as:

coreos/coreos-assembler#1597

There, we pass the Ignition config for the PXE boot via
`ignition.config.url`, but if the metal (no-op) fetcher appears earlier
than the `cmdline` fetcher, we get no config. And similarly for the
installed system when the no-op fetcher appears before the `system`
fetcher (which coreos-installer's `--ignition-file` leverages).

The likelihood of this happening increased in the v2.4.0 release due to
coreos#1002, which only gave us one try
to iterate over the correct provider first (at the `fetch` stage),
rather than every stage having a go at it.

Closes: coreos/coreos-assembler#1597

internal/exec/engine.go

@@ -272,20 +272,25 @@ func (e *Engine) acquireProviderConfig() (cfg types.Config, err error) {
 // is unavailable. This will also render the config (see renderConfig) before
 // returning.
 func (e *Engine) fetchProviderConfig() (types.Config, error) {
-	fetchers := map[string]providers.FuncFetchConfig{
-		"cmdline":               cmdline.FetchConfig,
-		"system":                system.FetchConfig,
-		e.PlatformConfig.Name(): e.PlatformConfig.FetchFunc(),
+	// note this is an array because iteration order is important; see comment
+	// block just above
+	fetchers := []struct {
+		name      string
+		fetchFunc providers.FuncFetchConfig
+	}{
+		{"cmdline", cmdline.FetchConfig},
+		{"system", system.FetchConfig},
+		{e.PlatformConfig.Name(), e.PlatformConfig.FetchFunc()},
 	}
 	var cfg types.Config
 	var r report.Report
 	var err error
 	var providerKey string
-	for key, fetcher := range fetchers {
-		cfg, r, err = fetcher(e.Fetcher)
+	for _, fetcher := range fetchers {
+		cfg, r, err = fetcher.fetchFunc(e.Fetcher)
 		if err != providers.ErrNoProvider {
 			// successful, or failed on another error
-			providerKey = key
+			providerKey = fetcher.name
 			break
 		}
 	}