Fix the openvino_enabled check in build-cpp.nu

<details><summary>Claude's draft</summary>

rattler-build passes the boolean context variable to the script as the
string "True", so comparing it to "true" never matched. The OpenVINO step
was skipped, build-ov was never built, and onnxruntime-ep-openvino failed
to copy libonnxruntime_providers_openvino on linux-64 and win-64.

Lowercase the value before comparing it. Verified with a local build of
linux_64_c_compiler_version15c_stdlib_version2.17cuda_compiler_versionNonecxx_compiler_version15suffix:
the plugin builds, the linking check passes without missing_dso_allowlist
(the library does not link libonnxruntime_providers_shared), and the
py3.11 test passes.

Resume this Claude session:
```
cd /home/mark/git/feedstock
claude --resume fdaba65c-5a43-4f26-a32f-d49b747fdbd7
```
</details>

Claude-Session: https://claude.ai/code/session_01R9WZp6UVaiXYyC2hWTNWZY

Author: hmaarrfk

recipe/build-cpp.nu

@@ -184,7 +184,7 @@ if $build_unit_tests == "ON" {
 # Configured separately so that build-ci, which the other outputs reuse, stays
 # identical to a build without it: onnxruntime-ep-openvino ships only the plugin
 # library, built against conda-forge's OpenVINO.
-if ($env.openvino_enabled? | default "false") == "true" {
+if ($env.openvino_enabled? | default "false" | str lowercase) == "true" {
     let ov_build_py_args = ($build_py_args
         | where {|a| $a not-in ["--build_wheel" "--enable_lto"]}
         | each {|a| if $a == "build-ci" { "build-ov" } else { $a }})