Transition uv to the correct platform (#122)

Currently, uv is selected for the exec platform. If it were used as part
of a build action, this would be correct, but as it is invoked by an
emitted shell script it is not. This causes it to fail when running
builds on one platform, but targeting another. This is the case, for
example, when running builds in a remote execution cluster with Linux
workers, but compiling for a Mac. This patch transitions uv to the
proper platform, allowing it to work in this situation.

uv/private/pip.bzl

diff --git a/uv/private/pip.bzl b/uv/private/pip.bzl
index 2b5d546..0dd7a90 100644
--- a/uv/private/pip.bzl
+++ b/uv/private/pip.bzl
@@ -1,6 +1,7 @@
 "uv based pip compile rules"
 
 load("@rules_python//python:defs.bzl", "PyRuntimeInfo")
+load(":transition_to_target.bzl", "transition_to_target")
 
 _PY_TOOLCHAIN = "@bazel_tools//tools/python:toolchain_type"
 
@@ -17,7 +18,7 @@
     "py3_runtime": attr.label(),
     "data": attr.label_list(allow_files = True),
     "uv_args": attr.string_list(default = _DEFAULT_ARGS),
-    "_uv": attr.label(default = "@multitool//tools/uv", executable = True, cfg = "exec"),
+    "_uv": attr.label(default = "@multitool//tools/uv", executable = True, cfg = transition_to_target),
 }
 
 def _python_version(py3_runtime):
@@ -70,7 +71,7 @@
         files = [ctx.file.requirements_in, ctx.file.requirements_txt] + ctx.files.data,
         transitive_files = py3_runtime.files,
     )
-    runfiles = runfiles.merge(ctx.attr._uv.default_runfiles)
+    runfiles = runfiles.merge(ctx.attr._uv[0].default_runfiles)
     return runfiles
 
 def _pip_compile_impl(ctx):

uv/private/transition_to_target.bzl

diff --git a/uv/private/transition_to_target.bzl b/uv/private/transition_to_target.bzl
new file mode 100644
index 0000000..0b4343c
--- /dev/null
+++ b/uv/private/transition_to_target.bzl
@@ -0,0 +1,16 @@
+"Helper rule to ensure that uv is resolved for the target architecture"
+
+def _transition_to_target_impl(settings, _attr):
+    return {
+        # String conversion is needed to prevent a crash with Bazel 6.x.
+        "//command_line_option:extra_execution_platforms": [
+            str(platform)
+            for platform in 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"],
+)

uv/private/venv.bzl

diff --git a/uv/private/venv.bzl b/uv/private/venv.bzl
index ccb855f..1cc298c 100644
--- a/uv/private/venv.bzl
+++ b/uv/private/venv.bzl
@@ -1,5 +1,7 @@
 "uv based venv generation"
 
+load(":transition_to_target.bzl", "transition_to_target")
+
 _PY_TOOLCHAIN = "@bazel_tools//tools/python:toolchain_type"
 
 def _uv_template(ctx, template, executable):
@@ -23,7 +25,7 @@
         files = [ctx.file.requirements_txt] + ctx.files.site_packages_extra_files,
         transitive_files = py_toolchain.py3_runtime.files,
     )
-    runfiles = runfiles.merge(ctx.attr._uv.default_runfiles)
+    runfiles = runfiles.merge(ctx.attr._uv[0].default_runfiles)
     return runfiles
 
 def _venv_impl(ctx):
@@ -39,7 +41,7 @@
         "destination_folder": attr.string(default = "venv"),
         "site_packages_extra_files": attr.label_list(default = [], doc = "Files to add to the site-packages folder inside the virtual environment. Useful for adding `sitecustomize.py` or `.pth` files", allow_files = True),
         "requirements_txt": attr.label(mandatory = True, allow_single_file = True),
-        "_uv": attr.label(default = "@multitool//tools/uv", executable = True, cfg = "exec"),
+        "_uv": attr.label(default = "@multitool//tools/uv", executable = True, cfg = transition_to_target),
         "_template": attr.label(default = "//uv/private:create_venv.sh", allow_single_file = True),
     },
     toolchains = [_PY_TOOLCHAIN],