pip install flash_attn==2.5.9.post1 --no-build-isolation doesn't work

Whenever I type in pip install flash_attn==2.5.9.post1 --no-build-isolation I get the following error:

(seedvr) U:\SeedVR>pip install flash_attn==2.5.9.post1 --no-build-isolation
Collecting flash_attn==2.5.9.post1
Using cached flash_attn-2.5.9.post1.tar.gz (2.6 MB)
Preparing metadata (setup.py) ... error
error: subprocess-exited-with-error

× python setup.py egg_info did not run successfully.
│ exit code: 1
╰─> [20 lines of output]
fatal: not a git repository (or any of the parent directories): .git
C:\Users\karyu\AppData\Local\Temp\pip-install-o5hwfarl\flash-attn_78ec49ae28a14677a124eca79185d543\setup.py:78: UserWarning: flash_attn was requested, but nvcc was not found. Are you sure your environment has nvcc available? If you're installing within a container from https://hub.docker.com/r/pytorch/pytorch, only images whose names contain 'devel' will provide nvcc.
warnings.warn(
Traceback (most recent call last):
File "", line 2, in
File "", line 35, in
File "C:\Users\karyu\AppData\Local\Temp\pip-install-o5hwfarl\flash-attn_78ec49ae28a14677a124eca79185d543\setup.py", line 134, in
CUDAExtension(
File "C:\Users\karyu\.conda\envs\seedvr\lib\site-packages\torch\utils\cpp_extension.py", line 1077, in CUDAExtension
library_dirs += library_paths(cuda=True)
File "C:\Users\karyu\.conda\envs\seedvr\lib\site-packages\torch\utils\cpp_extension.py", line 1211, in library_paths
paths.append(_join_cuda_home(lib_dir))
File "C:\Users\karyu\.conda\envs\seedvr\lib\site-packages\torch\utils\cpp_extension.py", line 2419, in _join_cuda_home
raise OSError('CUDA_HOME environment variable is not set. '
OSError: CUDA_HOME environment variable is not set. Please set it to your CUDA install root.

torch.__version__ = 2.3.0+cpu

[end of output]

note: This error originates from a subprocess, and is likely not a problem with pip.
error: metadata-generation-failed

× Encountered error while generating package metadata.
╰─> See above for output.

note: This is an issue with the package mentioned above, not pip.
hint: See above for details.

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CUDA is installed on my Windows 11 PC and the CUDA_HOME has been set correctly in the environment variables.