Linux install fails: Generating Runtime JSON

When installing from source (and following the instructions in the build readme), the following script fails:

```sh
mkdir -p build/linux_release
cd build/linux_release
cmake -DCMAKE_BUILD_TYPE=RelWithDebInfo ../..
make
```

With the following errors:
```
[ 94%] Built target openxr_runtime_list
[ 95%] Generating Runtime JSON [...]/OpenXR-SDK-Source/build/linux_release/src/tests/test_runtimes/openxr/1/active_runtime.json using -f [...]/OpenXR-SDK-Source/build/linux_release/src/tests/test_runtimes/openxr/1/active_runtime.json -l [...]/OpenXR-SDK-Source/build/linux_release/src/tests/test_runtimes/libtest_runtime.so -b
Traceback (most recent call last):
File "[...]/OpenXR-SDK-Source/src/scripts/generate_runtime_manifest.py", line 286, in
main(sys.argv[1:])
~~~~^^^^^^^^^^^^^^
File "[...]/OpenXR-SDK-Source/src/scripts/generate_runtime_manifest.py", line 61, in main
with open(output_file, 'w') as f:
~~~~^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: '[...]/OpenXR-SDK-Source/build/linux_release/src/tests/test_runtimes/openxr/1/active_runtime.json'
make[2]: *** [src/tests/test_runtimes/CMakeFiles/test_runtime.dir/build.make:74: src/tests/test_runtimes/openxr/1/active_runtime.json] Error 1
make[1]: *** [CMakeFiles/Makefile2:2060: src/tests/test_runtimes/CMakeFiles/test_runtime.dir/all] Error 2
make: *** [Makefile:146: all] Error 2
```
(I used [...] to sub out the path I cloned the project to)

I initially tried installing the arch linux AUR package `openxr-loader-git` and it failed with the same errors. I thought it was an issue with the arch-specific package, but I get the same issue from the cloned repo.

`[...]/OpenXR-SDK-Source/build/linux_release/src/tests/test_runtimes` only contains a Makefile and other cmake files. There is no openxr directory.

Also tried setting XR_RUNTIME_JSON and re-running, but the test seems to be using a hard-coded directory:
`export XR_RUNTIME_JSON=/home/USERNAME/.local/share/Steam/steamapps/common/SteamVR/steamxr_linux64.json`

Start with the build commands in the issue and inspect src/scripts/generate_runtime_manifest.py alongside the src/tests/test_runtimes CMake target. Check how the target chooses the output path and whether its parent directory is created before generation. Done means a clean make completes and the reported active_runtime.json is generated in the test runtime directory.

Written by the indexing model from the issue text.