Replace whoami with a more generic command

In our docker environment, the normal user does not have a name.
This results in the `whoami` command to fail which expects a name to print

Replace `whoami` with `id -u`, which print print the numeric id

I have no name!@7427761b8f4c:/tmp/d$ whoami
whoami: cannot find name for user ID 1000
I have no name!@7427761b8f4c:/tmp/d$ id
uid=1000 gid=0(root) groups=0(root)

Signed-off-by: Ritesh Raj Sarraf <ritesh.sarraf@collabora.com>

src/subprocess_test.cc

@@ -182,7 +182,7 @@ TEST_F(SubprocessTest, SetWithMulti) {
     "cmd /c echo hi",
     "cmd /c time /t",
 #else
-    "whoami",
+    "id -u",
     "pwd",
 #endif
   };