Direct log output to stderr and hide progress bar when redirected (#74)

After this change it is possible to differentiate between the log output
from solbuild itself, and the output from commands executed by solbuild.
This allows one to check the build result on the terminal and keep the
redirected stdout for checking the reason for the build result. This is
a follow-up of #54/#63.

Additionally, the progress bar is replaced by a log statement when
redirected.

For example:

```
$ sudo solbuild build package.yml -p unstable-x86_64 > /tmp/output.log
 ✓ > Downloading source uri=https://www.nano-editor.org/dist/v7/nano-7.2.tar.xz
 ✓ > Now starting build package=nano
 ✓ > Building succeeded
```

builder/source/simple.go

@@ -190,13 +190,35 @@ func (s *SimpleSource) download(destination string) error {
 			},
 		},
 	}
-
 	resp := client.Do(req)
 
-	// Setup our progress bar
+	// Show our progress bar
+	s.showProgress(resp)
+
+	if err := resp.Err(); err != nil {
+		slog.Error("Error downloading", "uri", s.URI, "err", err)
+
+		return err
+	}
+
+	return nil
+}
+
+func onTTY() bool {
+	s, _ := os.Stdout.Stat()
+
+	return s.Mode()&os.ModeCharDevice > 0
+}
+
+func (s *SimpleSource) showProgress(resp *grab.Response) {
+	if !onTTY() {
+		slog.Info("Downloading source", "uri", s.URI)
+	}
+
 	pbar := pb.Start64(resp.Size())
 	pbar.Set(pb.Bytes, true)
 	pbar.SetTemplateString(progressBarTemplate)
+	pbar.SetWriter(os.Stdout)
 
 	defer pbar.Finish()
 
@@ -212,12 +234,7 @@ func (s *SimpleSource) download(destination string) error {
 			// Ensure progressbar completes to 100%
 			pbar.SetCurrent(resp.BytesComplete())
 
-			if err := resp.Err(); err != nil {
-				slog.Error("Error downloading", "uri", s.URI, "err", err)
-				return err
-			}
-
-			return nil
+			return
 		}
 	}
 }

cli/log/log.go

@@ -38,20 +38,20 @@ func setLogger(h slog.Handler) {
 }
 
 func onTTY() bool {
-	s, _ := os.Stdout.Stat()
+	s, _ := os.Stderr.Stat()
 
 	return s.Mode()&os.ModeCharDevice > 0
 }
 
 func SetColoredLogger() {
-	setLogger(powerline.NewHandler(os.Stdout, &powerline.HandlerOptions{
+	setLogger(powerline.NewHandler(os.Stderr, &powerline.HandlerOptions{
 		Level:  &Level,
 		Colors: colors,
 	}))
 }
 
 func SetUncoloredLogger() {
-	setLogger(slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{
+	setLogger(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{
 		Level: &Level,
 	}))
 }