ci/prow-entrypoint: tell git repo under /src is safe

That directory is part of the image and so not owned by us. Likely the
base image used for the src container was updated recently to include
newer git.

ci/prow-entrypoint.sh

@@ -167,13 +167,18 @@ validate() {
     workdir="$(mktemp -d)"
     echo "Using $workdir as working directory"
 
+    # for `git config --global` below
+    export HOME=${workdir}
+
     # Figure out if we are running from the COSA image or directly from the Prow src image
     if [[ -d /src/github.com/openshift/os ]]; then
         cd "$workdir"
+        git config --global --add safe.directory /src/github.com/openshift/os
         git clone /src/github.com/openshift/os os
     elif [[ -d ./.git ]]; then
         srcdir="${PWD}"
         cd "$workdir"
+        git config --global --add safe.directory "${srcdir}"
         git clone "${srcdir}" os
     else
         echo "Could not found source directory"