meson: avoid bogus meson warning

With meson-0.52.0-1.module_f31+6771+f5d842eb.noarch I get:
src/test/meson.build:19: WARNING: Overriding previous value of environment variable 'PATH' with a new one

When we're using *prepend*, the whole point is to modify an existing variable,
so meson shouldn't warn. But let's set avoid the warning and shorten things by
setting the final value immediately.

src/test/meson.build

@@ -11,12 +11,11 @@ test_hashmap_ordered_c = custom_target(
 
 test_include_dir = include_directories('.')
 
-path = run_command('sh', ['-c', 'echo "$PATH"']).stdout()
+path = run_command('sh', ['-c', 'echo "$PATH"']).stdout().strip()
 test_env = environment()
 test_env.set('SYSTEMD_KBD_MODEL_MAP', kbd_model_map)
 test_env.set('SYSTEMD_LANGUAGE_FALLBACK_MAP', language_fallback_map)
-test_env.set('PATH', path)
-test_env.prepend('PATH', meson.build_root())
+test_env.set('PATH', '@0@:@1@'.format(meson.build_root(), path))
 
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