Add a concurrency lock for download/install against the same destination

combineDirTrees' merge logic assumes it's the only thing moving files
into a given target at a time; two `vintner download`/`install` runs
racing against the same --dest could otherwise interleave os.Rename
calls and corrupt the tree instead of erroring cleanly. Take an
exclusive, non-blocking flock(2) on the destination for the duration
of each run, so a second invocation fails immediately with a clear
message instead of silently colliding with the first.

internal/install/install.go

@@ -15,6 +15,7 @@ import (
 	"strings"
 
 	"github.com/Cheviiot/vintner/assets"
+	"github.com/Cheviiot/vintner/internal/lock"
 	"github.com/Cheviiot/vintner/internal/wineenv"
 )
 
@@ -34,6 +35,12 @@ func Install(dest, selfBinary string) error {
 		return fmt.Errorf("destination %q is not a directory", dest)
 	}
 
+	unlock, err := lock.Acquire(dest)
+	if err != nil {
+		return err
+	}
+	defer unlock()
+
 	// Targets are relative so the whole installed tree stays relocatable -
 	// moving or renaming dest doesn't break these symlinks the way an
 	// absolute target baked in at install time would.