chore: rename release artifacts to match rust convention (#285)

### Type of change

- Refactor (a code change that neither fixes a bug or adds a new
feature)

### Test plan

- Manual testing; please provide instructions so we can reproduce:

```
bazel build py/tools/unpack_bin:linux
Target //py/tools/unpack_bin:linux up-to-date:
  bazel-bin/py/tools/unpack_bin/unpack-aarch64-unknown-linux-gnu
  bazel-bin/py/tools/unpack_bin/unpack-aarch64-unknown-linux-gnu.sha256
  bazel-bin/py/tools/unpack_bin/unpack-x86_64-unknown-linux-gnu
  bazel-bin/py/tools/unpack_bin/unpack-x86_64-unknown-linux-gnu.sha256
```

Author: alexeagle

tools/release/defs.bzl

@@ -1,9 +1,17 @@
 "Make releases for platforms supported by rules_py"
 
-load("@aspect_bazel_lib//lib:transitions.bzl", "platform_transition_filegroup")
 load("@aspect_bazel_lib//lib:copy_file.bzl", "copy_file")
+load("@aspect_bazel_lib//lib:transitions.bzl", "platform_transition_filegroup")
 load("@aspect_bazel_lib//tools/release:hashes.bzl", "hashes")
 
+def _map_os_to_triple(os):
+    if os == "linux":
+        # TODO: when we produce musl-linked binaries, this should change to unknown-linux-musl
+        return "unknown-linux-gnu"
+    if os == "macos":
+        return "apple-darwin"
+    fail("unrecognized os", os)
+
 # buildozer: disable=function-docstring
 def multi_arch_rust_binary_release(name, src, os, archs = ["aarch64", "x86_64"], **kwargs):
     outs = []
@@ -16,7 +24,8 @@ def multi_arch_rust_binary_release(name, src, os, archs = ["aarch64", "x86_64"],
             target_compatible_with = ["@platforms//os:{}".format(os)],
         )
 
-        artifact = "{}-{}-{}".format(bin, os, arch)
+        # Artifact naming follows typical Rust "triples" convention.
+        artifact = "{}-{}-{}".format(bin, arch, _map_os_to_triple(os))
         outs.append(artifact)
         copy_file(
             name = "copy_{}_{}_{}".format(bin, os, arch),