vince@guixsd ~$ tree ~
/home/vince
├── guix-packages
│   ├── patches
│   │   └── sinit-build-fix.patch
│   └── sinit.scm
└── sinit-1.0.tar.bz2

2 directories, 3 files
vince@guixsd ~$ echo $GUIX_PACKAGE_PATH
/home/vince/guix-packages
vince@guixsd ~$ guix build sinit
guix build: error: sinit-build-fix.patch: patch not found