Linux cross-RBE fix: allow pkg_tar callers to override the helper executable.

This is the rules_pkg rule-side companion to pkg_tar_host_tool.patch. MongoDB
uses the new attribute only for install archives whose actions are constrained
to the client host.

Applied via `single_version_override(patches=...)` on rules_pkg in
//MODULE.bazel. Candidate for upstreaming to bazelbuild/rules_pkg.

--- a/pkg/private/tar/tar.bzl
+++ b/pkg/private/tar/tar.bzl
@@ -192,12 +192,13 @@
         transitive = mapping_context.file_deps,
     )
-
+
+    build_tar = ctx.executable.build_tar_tool or ctx.executable._build_tar
     ctx.actions.run(
         mnemonic = "PackageTar",
         progress_message = "Writing: %s" % output_file.path,
         inputs = inputs,
         tools = [ctx.executable.compressor] if ctx.executable.compressor else [],
-        executable = ctx.executable._build_tar,
+        executable = build_tar,
         arguments = [args],
         outputs = [output_file],
         env = {
@@ -326,6 +327,12 @@
         # Is --stamp set on the command line?
         # TODO(https://github.com/bazelbuild/rules_pkg/issues/340): Remove this.
         "private_stamp_detect": attr.bool(default = False),
-
+
+        "build_tar_tool": attr.label(
+            doc = "Optional build_tar executable override.",
+            cfg = "exec",
+            executable = True,
+            allow_files = True,
+        ),
         # Implicit dependencies.
         "_build_tar": attr.label(