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 valac transpiler needs search paths for the host machine
(specifically, the `--vapidir` default paths).

This reverts part of af846a1.

Author: oleavr

mesonbuild/compilers/detect.py

@@ -968,7 +968,7 @@ def detect_cython_compiler(env: 'Environment', for_machine: MachineChoice) -> Co
     raise EnvironmentException('Unreachable code (exception to make mypy happy)')
 
 def detect_vala_compiler(env: 'Environment', for_machine: MachineChoice) -> Compiler:
-    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: