clang does not get detected if it has a non-standard path

[dstolfa]

Running

./cheribuild.py run-riscv64-purecap -d

results in

Checking /home/ds815/.config/cheribuild.json since /home/ds815/cheribuild/cheribuild.json doesn't exist
Warning: Configuration file /home/ds815/.config/cheribuild.json does not exist, using only command line arguments.
Sources will be stored in /home/ds815/cheri
Build artifacts will be stored in /home/ds815/cheri/output
cd /home/ds815/cheribuild && git fetch
Will execute the following targets:
   qemu
   llvm-native
   cheribsd-riscv64-purecap
   gdb-riscv64-hybrid-for-purecap-rootfs
   bbl-baremetal-riscv64-purecap
   disk-image-riscv64-purecap
   run-riscv64-purecap
Could not find `gmake` command, assuming `make` is GNU make
Warning: Could not detect compiler info for /c/compiler/is/missing - output was b"[Errno 2] No such file or directory: '/c/compiler/is/missing'"
Fatal error: Dependency for llvm-native missing: /c/compiler/is/missing version 0.0.0 is not supported. Clang version 3.8 or newer is required.
Possible solution: Try installing clang 3.8 or newer using your system package manager

however, running

which clang
/home/ds815/.guix-profile/bin/clang