tests/common/44: Prepend to PKG_CONFIG_PATh instead of replacing it

Which is required to make this test work when PKG_CONFIG_PATH is
required to get a working system, such as on nixos

test cases/common/44 pkgconfig-gen/meson.build

@@ -38,9 +38,15 @@ pkgg.generate(
   libraries_private : [lib, '-lz'],
 )
 
-test('pkgconfig-validation', pkgconfig,
+env = environment()
+env.prepend('PKG_CONFIG_PATH', meson.current_build_dir() / 'meson-private')
+
+test(
+  'pkgconfig-validation',
+  pkgconfig,
   args: ['--validate', 'simple'],
-  env: [ 'PKG_CONFIG_PATH=' + meson.current_build_dir() + '/meson-private' ])
+  env : env,
+)
 
 answerlib = shared_library('answer', 'answer.c')
 

test cases/common/44 pkgconfig-gen/test.json

@@ -18,10 +18,10 @@
   ],
   "stdout": [
     {
-      "line": "test cases/common/44 pkgconfig-gen/meson.build:158: WARNING: Project targeting '>=0.60.0' but tried to use feature introduced in '0.62.0': pkgconfig.generate implicit variable for builtin directories."
+      "line": "test cases/common/44 pkgconfig-gen/meson.build:164: WARNING: Project targeting '>=0.60.0' but tried to use feature introduced in '0.62.0': pkgconfig.generate implicit variable for builtin directories."
     },
     {
-      "line": "test cases/common/44 pkgconfig-gen/meson.build:164: WARNING: Project targeting '>=0.60.0' but tried to use feature introduced in '0.62.0': pkgconfig.generate implicit variable for builtin directories.",
+      "line": "test cases/common/44 pkgconfig-gen/meson.build:170: WARNING: Project targeting '>=0.60.0' but tried to use feature introduced in '0.62.0': pkgconfig.generate implicit variable for builtin directories.",
       "count": 0
     },
     {