graphics/mesa-dri: respect LLVM_DEFAULT after e70ff172c205

Meson probes FreeBSD suffixes[1] for llvm-config but ends up selecting
the last match instead of the unsuffixed version. So, replace BINARY_ALIAS
with recently introduced[2] ad hoc environ(7) variable.

[1] https://github.com/mesonbuild/meson/blob/0.64.1/mesonbuild/environment.py#L159-L177
[2] https://github.com/mesonbuild/meson/commit/2c1ac8ac1ddb

Approved by: manu (on dev-commits-ports-main@ list)