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.

(cherry picked from commit cbe8049)

Related: #2030027

src/test/meson.build

@@ -10,12 +10,11 @@ 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))
 
 ############################################################