refactor: js_image_layer to output with .tar extension when uncompressed (#2585)

Fixes #2584

### Changes are visible to end-users: yes

- Searched for relevant documentation and updated as needed: yes
- Breaking change (forces users to change their own code or config): no
- Suggested release notes appear below: no

### Test plan

`bazel test //...` passes

Author: fa93hws

js/private/js_image_layer.bzl

@@ -568,7 +568,7 @@ def _js_image_layer_impl(ctx):
     output_groups = dict()
     compress = "" if ctx.attr.compression == "none" else ctx.attr.compression
     for typ, mtree, unused_inputs in layer_groups_gen:
-        ext = tar_lib.common.compression_to_extension[compress] if compress else ""
+        ext = tar_lib.common.compression_to_extension[compress] if compress else ".tar"
         output = ctx.actions.declare_file("%s_%s%s" % (ctx.label.name, typ, ext))
 
         # add the layer group to outputgroupinfo and defaultinfo

js/private/test/image/checksum_test.expected

@@ -1,4 +1,4 @@
-b1b723afb1aac593e55cc5050909103650a13306d1f0366c5405dcc6cf73461f  js/private/test/image/cksum_node
-052600f3a82ab6a4cc12cab7384971c960f9c589fdbfcf21bca563c36ff7d16e  js/private/test/image/cksum_package_store_3p
-971f291232f3ab63aff37fb66c96fbf0eddc05ea9564b9673d0d2c9bfe958994  js/private/test/image/cksum_package_store_1p
-febf95a6d554c9bda3f0515bfd5ef273ac67d31c231d8162beaef8c4b7bc72f3  js/private/test/image/cksum_node_modules
+b1b723afb1aac593e55cc5050909103650a13306d1f0366c5405dcc6cf73461f  js/private/test/image/cksum_node.tar
+052600f3a82ab6a4cc12cab7384971c960f9c589fdbfcf21bca563c36ff7d16e  js/private/test/image/cksum_package_store_3p.tar
+971f291232f3ab63aff37fb66c96fbf0eddc05ea9564b9673d0d2c9bfe958994  js/private/test/image/cksum_package_store_1p.tar
+febf95a6d554c9bda3f0515bfd5ef273ac67d31c231d8162beaef8c4b7bc72f3  js/private/test/image/cksum_node_modules.tar