feat: add CIBUILDWHEEL_BUILD_IDENTIFIER environment variable (#2872)

* feat: add CIBUILDWHEEL_BUILD_IDENTIFIER environment variable

Set `CIBUILDWHEEL_BUILD_IDENTIFIER` to the current build identifier
(e.g. `cp311-manylinux_x86_64`) in the environment for all per-build
steps: `before_build`, the build itself, `repair_command`,
`before_test`, and `test_command`.

This allows scripts and commands to inspect which build is currently
running, which is useful for e.g. writing per-build output files:

    CIBW_TEST_COMMAND='pytest --junit-xml=results-$CIBUILDWHEEL_BUILD_IDENTIFIER.xml'

The variable is set after the user's environment overrides are applied
and is only available for per-build steps (not `before_all`, where no
single identifier applies). All six platforms are covered: linux, macOS,
Windows, pyodide, Android, and iOS.

Closes #944, closes #2750

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Assisted-by: OpenCode:claude-sonnet-4.6

* fix: address review comment moving this for Android

Assisted-by: OpenCode:glm-5.1
Signed-off-by: Henry Schreiner <henryfs@princeton.edu>

---------

Signed-off-by: Henry Schreiner <henryfs@princeton.edu>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

cibuildwheel/platforms/android.py

@@ -140,6 +140,7 @@ def build(options: Options, tmp_path: Path) -> None:
             build_path.mkdir()
             python_dir = setup_target_python(config, build_path)
             build_env, android_env = setup_env(config, build_options, build_path, python_dir)
+
             state = BuildState(
                 config, build_options, build_path, python_dir, build_env, android_env
             )
@@ -228,6 +229,7 @@ def setup_env(
 
     # Apply custom environment variables, and check environment is still valid
     build_env = build_options.environment.as_dictionary(build_env)
+    build_env["CIBUILDWHEEL_BUILD_IDENTIFIER"] = config.identifier
     build_env["PIP_DISABLE_PIP_VERSION_CHECK"] = "1"
     for command in ["python"] if use_uv else ["python", "pip"]:
         command_path = call("which", command, env=build_env, capture_stdout=True).strip()