find_program: Don't leak mutable array in get_command()

In several places in the gnome module, we are getting an array from this
and directly appending to it which changes the original command array
ever since we started caching the find_program results.

We urgently need to move to immutable types for all object properties.

Reported by Alexandor Larsson on IRC.

mesonbuild/dependencies.py

@@ -476,7 +476,7 @@ class ExternalProgram():
         return self.fullpath[0] is not None
 
     def get_command(self):
-        return self.fullpath
+        return self.fullpath[:]
 
     def get_name(self):
         return self.name