Use default python in the base image (#1589)

* use default python in the base image

Signed-off-by: Kevin Su <pingsutw@apache.org>

* nit

Signed-off-by: Kevin Su <pingsutw@apache.org>

* lint fix

Signed-off-by: Kevin Su <pingsutw@apache.org>

* nit

Signed-off-by: Kevin Su <pingsutw@apache.org>

---------

Signed-off-by: Kevin Su <pingsutw@apache.org>

Author: pingsutw

pkg/lang/ir/v1/system.go

@@ -256,16 +256,17 @@ func (g *generalGraph) compileLanguage(root llb.State) (llb.State, error) {
 
 func (g *generalGraph) compileLanguagePackages(root llb.State) llb.State {
 	packs := []llb.State{}
-	pack := root
+
+	// Use default python in the base image if install.python() is not specified.
+	index := g.compilePyPIIndex(root)
+	pack := g.compilePyPIPackages(index)
+	if g.CondaConfig != nil {
+		channel := g.compileCondaChannel(pack)
+		pack = g.compileCondaPackages(channel)
+	}
+
 	for _, language := range g.Languages {
 		switch language.Name {
-		case "python":
-			index := g.compilePyPIIndex(root)
-			pack = g.compilePyPIPackages(index)
-			if g.CondaConfig != nil {
-				channel := g.compileCondaChannel(pack)
-				pack = g.compileCondaPackages(channel)
-			}
 		case "r":
 			pack = g.installRPackages(root)
 		case "julia":