always set DESTDIR via the environment for external projects

It doesn't matter whether it is make or not, because make, too, supports
setting it via the env. This reduces the use of special cases in the
code.

mesonbuild/scripts/externalproject.py

@@ -65,10 +65,7 @@ class ExternalProject:
 
         install_cmd = self.make.copy()
         install_env = {}
-        if is_make:
-            install_cmd.append(f'DESTDIR={self.install_dir}')
-        else:
-            install_env['DESTDIR'] = self.install_dir
+        install_env['DESTDIR'] = self.install_dir
         install_cmd.append('install')
         rc = self._run('install', install_cmd, install_env)
         if rc != 0: