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>
diff --git a/src/subprocess_test.cc b/src/subprocess_test.cc
index 0a8c206..6e487db 100644
--- a/src/subprocess_test.cc
+++ b/src/subprocess_test.cc
@@ -182,7 +182,7 @@
     "cmd /c echo hi",
     "cmd /c time /t",
 #else
-    "whoami",
+    "id -u",
     "pwd",
 #endif
   };