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/pyodide.py

@@ -391,6 +391,7 @@ def build(options: Options, tmp_path: Path) -> None:
                 environment=build_options.environment,
                 user_pyodide_version=build_options.pyodide_version,
             )
+            env["CIBUILDWHEEL_BUILD_IDENTIFIER"] = config.identifier
             pip_version = get_pip_version(env)
             # The Pyodide command line runner mounts all directories in the host
             # filesystem into the Pyodide file system, except for the custom