build(tensorrt): scope JP7 build changes to reduce blast radius

Gate the docker/main/Dockerfile pip --ignore-installed workaround behind a
PIP_IGNORE_INSTALLED build arg that is empty for every stock image and set
only for the JP7 (Ubuntu 24.04 noble) TensorRT base. Non-JP7 images now build
a byte-identical command. Revert the ov-converter change entirely since that
stage is always debian:12, never noble.

Drop the incidental shell re-quoting in the trt-model-prepare run script,
keeping only the functional TensorRT-major-aware runtime library check
(.so.8 on JP6, .so.10 on JP7; libnvparsers removed in TRT 10).

.github/workflows/ci.yml

@@ -131,6 +131,7 @@ jobs:
           TENSORRT_PYTHON_BRANCH: auto
           L4T_APT_RELEASE: r39.2
           JETSON_SOC_REPO: som
+          PIP_IGNORE_INSTALLED: "--ignore-installed"
         uses: docker/bake-action@v7
         with:
           source: .