Stop printing Ivy credentials during coverity scan

Rely on the credentials being in the environment rather than explicitly
passing them. Add a comment to document the requirement.

See also: 0d840bb2

concourse/scripts/scan_with_coverity.bash

@@ -31,7 +31,9 @@ function generate_build_number() {
 
 function make_sync_tools() {
   pushd gpdb_src/gpAux
-    make IVYREPO_HOST="${IVYREPO_HOST}" IVYREPO_REALM="${IVYREPO_REALM}" IVYREPO_USER="${IVYREPO_USER}" IVYREPO_PASSWD="${IVYREPO_PASSWD}" sync_tools
+    # Requires these variables in the env:
+    # IVYREPO_HOST IVYREPO_REALM IVYREPO_USER IVYREPO_PASSWD
+    make sync_tools
     # We have compiled LLVM with native zlib on CentOS6 and not from
     # the zlib downloaded from artifacts.  Therefore, remove the zlib
     # downloaded from artifacts in order to use the native zlib.