Use a transition to resolve the correct versions of crane and yq

Bazel doesn't provide a very elegant way to specify that a toolchain
should be resolved in such a way that it can be executed on a *target*
platform:

bazelbuild/bazel#19645

To work around this, we can move the toolchains into a small helper rule
named oci_push_tools(). We can use an outgoing edge transition on the
oci_push() side to set --extra_execution_platforms equal to --platforms,
thereby causing the toolchains to be resolved the way we want.

Though I think using transitions for this is pretty slick, it's a shame
that this has to be done by patching up command line options.

oci/private/BUILD.bazel

@@ -1,4 +1,5 @@
 load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
+load(":push.bzl", "oci_push_tools")
 
 exports_files(
     glob(["*.bzl"]),
@@ -108,3 +109,8 @@ bzl_library(
         "@bazel_skylib//lib:versions",
     ],
 )
+
+oci_push_tools(
+    name = "oci_push_tools",
+    visibility = ["//visibility:public"],
+)

oci/private/push.bzl

@@ -98,6 +98,19 @@ oci_push(
 ```
 """
 
+# Helper rule for ensuring that the crane and yq toolchains are actually
+# resolved for the architecture we are targeting.
+def _transition_to_target_impl(settings, attr):
+    return {
+        "//command_line_option:extra_execution_platforms": settings["//command_line_option:platforms"],
+    }
+
+_transition_to_target = transition(
+    implementation = _transition_to_target_impl,
+    inputs = ["//command_line_option:platforms"],
+    outputs = ["//command_line_option:extra_execution_platforms"],
+)
+
 _attrs = {
     "image": attr.label(
         allow_single_file = True,
@@ -128,15 +141,19 @@ _attrs = {
         default = "push.sh.tpl",
         allow_single_file = True,
     ),
+    "_tools": attr.label(
+        cfg = _transition_to_target,
+        default = "oci_push_tools",
+    ),
     "_windows_constraint": attr.label(default = "@platforms//os:windows"),
 }
 
 def _quote_args(args):
     return ["\"{}\"".format(arg) for arg in args]
 
 def _impl(ctx):
-    crane = ctx.toolchains["@rules_oci//oci:crane_toolchain_type"]
-    yq = ctx.toolchains["@aspect_bazel_lib//lib:yq_toolchain_type"]
+    crane = ctx.attr._tools[0].crane
+    yq = ctx.attr._tools[0].yq
 
     if ctx.attr.repository and ctx.attr.repository_file:
         fail("must specify exactly one of 'repository_file' or 'repository'")
@@ -195,6 +212,16 @@ oci_push = rule(
     doc = _DOC,
     implementation = oci_push_lib.implementation,
     attrs = oci_push_lib.attrs,
-    toolchains = oci_push_lib.toolchains,
     executable = True,
 )
+
+def _oci_push_tools(ctx):
+    return struct(
+        crane = ctx.toolchains["@rules_oci//oci:crane_toolchain_type"],
+        yq = ctx.toolchains["@aspect_bazel_lib//lib:yq_toolchain_type"],
+    )
+
+oci_push_tools = rule(
+    implementation = _oci_push_tools,
+    toolchains = oci_push_lib.toolchains,
+)