filament installs spirv-tools which conflicts with the external package spirv-tools

**Describe the bug**
For example, the file /usr/local/bin/spirv-as is installed that conflicts with the package:
```
Installing filament-1.62.1...
pkg-static: filament-1.62.1 conflicts with spirv-tools-2025.2.r1 (installs files into the same place). Problematic file: /usr/local/bin/spirv-as
*** Error code 1
```

**To Reproduce**
1. have the spirv-tools package installed
2. build and install filament
3. observe that the file bin/spirv-as is overwritten

**Expected behavior**
No such conflict: filament should use an external package when available or when enabled by the user.

**Screenshots**
n/a

**Logs**
n/a

**Desktop (please complete the following information):**
- OS: FreeBSD 14.3
- GPU: n/a
- Backend: all

**Smartphone (please complete the following information):**
n/a

**Additional context**
Version: 1.62.1