compilers: Detect Vala compiler for the requested machine

Instead of unconditionally detecting it for the build machine.
The cross-file may want to specify a particular compiler.

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: