Tests: use t.Setenv to set env vars (#69516)

This commit replaces `os.Setenv` with `t.Setenv` in tests. The
environment variable is automatically restored to its original value
when the test and all its subtests complete.

Reference: https://pkg.go.dev/testing#T.Setenv

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

pkg/build/env/fallback_test.go

@@ -135,9 +135,6 @@ func setEnv(t *testing.T, key, value string) string {
 	t.Helper()
 	os.Clearenv()
 
-	err := os.Setenv(key, value)
-	if err != nil {
-		require.NoError(t, err)
-	}
+	t.Setenv(key, value)
 	return key
 }