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Fix `IRIS_ROOT` detection for nested project #343

Job	Run time
changes	4s
[C++23] Clang 22 (Release) / Dependencies	32s
[C++26] MSVC 2026 (Release) / Dependencies	36s
[C++26] GCC 15 (Release) / Dependencies	21s
[C++23] GCC 15 (Debug) / Dependencies	15s
[C++23] MSVC 2026 (Release) / Dependencies	44s
[C++26] MSVC 2026 (Debug) / Dependencies	34s
[C++26] Clang 22 (Release) / Dependencies	29s
[C++26] Clang 22 (Debug) / Dependencies	26s
[C++23] GCC 15 (Release) / Dependencies	13s
[C++23] Clang 22 (Debug) / Dependencies	40s
[C++26] GCC 15 (Debug) / Dependencies	17s
[C++23] MSVC 2026 (Debug) / Dependencies	40s
[C++23] GCC 15 (Debug) / iris	1m 56s
[C++23] GCC 15 (Release) / iris	3m 13s
[C++26] GCC 15 (Release) / iris	2m 10s
[C++23] Clang 22 (Release) / iris	3m 3s
[C++26] GCC 15 (Debug) / iris	1m 33s
[C++26] MSVC 2026 (Debug) / iris	3m 10s
[C++26] MSVC 2026 (Release) / iris	3m 34s
[C++23] Clang 22 (Debug) / iris	2m 23s
[C++26] Clang 22 (Release) / iris	3m 29s
[C++26] Clang 22 (Debug) / iris	1m 54s
[C++23] MSVC 2026 (Debug) / iris	2m 51s
[C++23] MSVC 2026 (Release) / iris	4m 11s
run-notifier	6s
	39m 24s