add pyexec root to `unrealsdk.env` #64

Job	Run time
build-ubuntu (clang-cross-release)	8m 36s
clang-format	10s
build-ubuntu (llvm-mingw-release)	3m 38s
clang-tidy (clang-release)	3m 11s
build-windows (clang-release)	9m 9s
build-ubuntu (mingw-release)	10m 13s
build-windows (msvc-release)	2m 59s
pyright	12s
ruff	7s
spelling	6s
release-dev-ci	0s
release-draft-full	0s
	38m 21s

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