`extractHeadlingArgsFromNode` incorrectly uses host `BuiltinArgDefaults`

extractHeadlingArgsFromNode creates headling args using a new temporary builder. However, if the current builder's BuiltinArgDefaults differ from the host's defaults (such as when passing --platform to buildah, which overrides TARGET* args after calling NewBuilder()), the resulting headling args contain the wrong values.

The function should instead copy the current builder's BuiltinArgDefaults into the temporary builder before trying to construct the headling args. The following patch should fix this:

--- a/builder.go
+++ b/builder.go
@@ -6,6 +6,7 @@ import (
        "fmt"
        "io"
        "log"
+       "maps"
        "os"
        "path/filepath"
        "runtime"
@@ -418,6 +419,7 @@ func (b *Builder) extractHeadingArgsFromNode(node *parser.Node) error {

        // Use a separate builder to evaluate the heading args
        tempBuilder := NewBuilder(b.UserArgs)
+       maps.Copy(tempBuilder.BuiltinArgDefaults, b.BuiltinArgDefaults)

        // Built-in ARGs are declared implicitly in the heading and should be resolvable in its scope
        for k, v := range tempBuilder.BuiltinArgDefaults {

The test cases in https://github.com/podman-container-tools/buildah/issues/6964 pass after making this change.

Start in builder.go at extractHeadingArgsFromNode, where a temporary builder evaluates heading arguments. Check the current and temporary BuiltinArgDefaults, then verify the behavior against the test cases referenced in buildah issue 6964. Done means heading arguments use the current builder's defaults, including values changed by --platform.