compilers: Detect Vala compiler for the requested machine

Instead of unconditionally detecting it for the build machine.
The rationale for making this use the build machine was wrong.
The Vala compiler needs search paths for the host machine
(specifically, the `--vapidir` default paths).

This reverts part of af846a1.

mesonbuild/compilers/detect.py

@@ -949,7 +949,7 @@ def detect_cython_compiler(env: 'Environment', for_machine: MachineChoice) -> Co
 
 def detect_vala_compiler(env: 'Environment', for_machine: MachineChoice) -> Compiler:
     from .vala import ValaCompiler
-    exelist = env.lookup_binary_entry(MachineChoice.BUILD, 'vala')
+    exelist = env.lookup_binary_entry(for_machine, 'vala')
     is_cross = env.is_cross_build(for_machine)
     info = env.machines[for_machine]
     if exelist is None: